diff --git a/src/Prometheus/Storage/APC.php b/src/Prometheus/Storage/APC.php
index b608e49..fb747ce 100644
--- a/src/Prometheus/Storage/APC.php
+++ b/src/Prometheus/Storage/APC.php
@@ -5,7 +5,6 @@
 
 
 use Prometheus\MetricFamilySamples;
-use RuntimeException;
 
 class APC implements Adapter
 {
@@ -26,19 +25,19 @@ public function updateHistogram(array $data)
     {
         // Initialize the sum
         $sumKey = $this->histogramBucketValueKey($data, 'sum');
-        $new = apcu_add($sumKey, $this->toInteger(0));
+        $new = apc_add($sumKey, $this->toInteger(0));
 
         // If sum does not exist, assume a new histogram and store the metadata
         if ($new) {
-            apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+            apc_store($this->metaKey($data), json_encode($this->metaData($data)));
         }
 
         // Atomically increment the sum
         // Taken from https://github.com/prometheus/client_golang/blob/66058aac3a83021948e5fb12f1f408ff556b9037/prometheus/value.go#L91
         $done = false;
         while (!$done) {
-            $old = apcu_fetch($sumKey);
-            $done = apcu_cas($sumKey, $old, $this->toInteger($this->fromInteger($old) + $data['value']));
+            $old = apc_fetch($sumKey);
+            $done = apc_cas($sumKey, $old, $this->toInteger($this->fromInteger($old) + $data['value']));
         }
 
         // Figure out in which bucket the observation belongs
@@ -51,42 +50,42 @@ public function updateHistogram(array $data)
         }
 
         // Initialize and increment the bucket
-        apcu_add($this->histogramBucketValueKey($data, $bucketToIncrease), 0);
-        apcu_inc($this->histogramBucketValueKey($data, $bucketToIncrease));
+        apc_add($this->histogramBucketValueKey($data, $bucketToIncrease), 0);
+        apc_inc($this->histogramBucketValueKey($data, $bucketToIncrease));
     }
 
     public function updateGauge(array $data)
     {
         $valueKey = $this->valueKey($data);
         if ($data['command'] == Adapter::COMMAND_SET) {
-            apcu_store($valueKey, $this->toInteger($data['value']));
-            apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+            apc_store($valueKey, $this->toInteger($data['value']));
+            apc_store($this->metaKey($data), json_encode($this->metaData($data)));
         } else {
-            $new = apcu_add($valueKey, $this->toInteger(0));
+            $new = apc_add($valueKey, $this->toInteger(0));
             if ($new) {
-                apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+                apc_store($this->metaKey($data), json_encode($this->metaData($data)));
             }
             // Taken from https://github.com/prometheus/client_golang/blob/66058aac3a83021948e5fb12f1f408ff556b9037/prometheus/value.go#L91
             $done = false;
             while (!$done) {
-                $old = apcu_fetch($valueKey);
-                $done = apcu_cas($valueKey, $old, $this->toInteger($this->fromInteger($old) + $data['value']));
+                $old = apc_fetch($valueKey);
+                $done = apc_cas($valueKey, $old, $this->toInteger($this->fromInteger($old) + $data['value']));
             }
         }
     }
 
     public function updateCounter(array $data)
     {
-        $new = apcu_add($this->valueKey($data), 0);
+        $new = apc_add($this->valueKey($data), 0);
         if ($new) {
-            apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+            apc_store($this->metaKey($data), json_encode($this->metaData($data)));
         }
-        apcu_inc($this->valueKey($data), $data['value']);
+        apc_inc($this->valueKey($data), $data['value']);
     }
 
     public function flushAPC()
     {
-       apcu_clear_cache();
+        apc_clear_cache('user');
     }
 
     /**
@@ -104,13 +103,7 @@ private function metaKey(array $data)
      */
     private function valueKey(array $data)
     {
-        return implode(':', array(
-            self::PROMETHEUS_PREFIX,
-            $data['type'],
-            $data['name'],
-            $this->encodeLabelValues($data['labelValues']),
-            'value'
-        ));
+        return implode(':', array(self::PROMETHEUS_PREFIX, $data['type'], $data['name'], json_encode($data['labelValues']), 'value'));
     }
 
     /**
@@ -119,14 +112,7 @@ private function valueKey(array $data)
      */
     private function histogramBucketValueKey(array $data, $bucket)
     {
-        return implode(':', array(
-            self::PROMETHEUS_PREFIX,
-            $data['type'],
-            $data['name'],
-            $this->encodeLabelValues($data['labelValues']),
-            $bucket,
-            'value'
-        ));
+        return implode(':', array(self::PROMETHEUS_PREFIX, $data['type'], $data['name'], json_encode($data['labelValues']), $bucket, 'value'));
     }
 
     /**
@@ -148,7 +134,7 @@ private function metaData(array $data)
     private function collectCounters()
     {
         $counters = array();
-        foreach (new \APCUIterator('/^prom:counter:.*:meta/') as $counter) {
+        foreach (new \APCIterator('user', '/^prom:counter:.*:meta/') as $counter) {
             $metaData = json_decode($counter['value'], true);
             $data = array(
                 'name' => $metaData['name'],
@@ -156,13 +142,13 @@ private function collectCounters()
                 'type' => $metaData['type'],
                 'labelNames' => $metaData['labelNames'],
             );
-            foreach (new \APCUIterator('/^prom:counter:' . $metaData['name'] . ':.*:value/') as $value) {
+            foreach (new \APCIterator('user', '/^prom:counter:' . $metaData['name'] . ':.*:value/') as $value) {
                 $parts = explode(':', $value['key']);
                 $labelValues = $parts[3];
                 $data['samples'][] = array(
                     'name' => $metaData['name'],
                     'labelNames' => array(),
-                    'labelValues' => $this->decodeLabelValues($labelValues),
+                    'labelValues' => json_decode($labelValues),
                     'value' => $value['value']
                 );
             }
@@ -178,7 +164,7 @@ private function collectCounters()
     private function collectGauges()
     {
         $gauges = array();
-        foreach (new \APCUIterator('/^prom:gauge:.*:meta/') as $gauge) {
+        foreach (new \APCIterator('user', '/^prom:gauge:.*:meta/') as $gauge) {
             $metaData = json_decode($gauge['value'], true);
             $data = array(
                 'name' => $metaData['name'],
@@ -186,13 +172,13 @@ private function collectGauges()
                 'type' => $metaData['type'],
                 'labelNames' => $metaData['labelNames'],
             );
-            foreach (new \APCUIterator('/^prom:gauge:' . $metaData['name'] . ':.*:value/') as $value) {
+            foreach (new \APCIterator('user', '/^prom:gauge:' . $metaData['name'] . ':.*:value/') as $value) {
                 $parts = explode(':', $value['key']);
                 $labelValues = $parts[3];
                 $data['samples'][] = array(
                     'name' => $metaData['name'],
                     'labelNames' => array(),
-                    'labelValues' => $this->decodeLabelValues($labelValues),
+                    'labelValues' => json_decode($labelValues),
                     'value' => $this->fromInteger($value['value'])
                 );
             }
@@ -209,7 +195,7 @@ private function collectGauges()
     private function collectHistograms()
     {
         $histograms = array();
-        foreach (new \APCUIterator('/^prom:histogram:.*:meta/') as $histogram) {
+        foreach (new \APCIterator('user', '/^prom:histogram:.*:meta/') as $histogram) {
             $metaData = json_decode($histogram['value'], true);
             $data = array(
                 'name' => $metaData['name'],
@@ -223,7 +209,7 @@ private function collectHistograms()
             $data['buckets'][] = '+Inf';
 
             $histogramBuckets = array();
-            foreach (new \APCUIterator('/^prom:histogram:' . $metaData['name'] . ':.*:value/') as $value) {
+            foreach (new \APCIterator('user', '/^prom:histogram:' . $metaData['name'] . ':.*:value/') as $value) {
                 $parts = explode(':', $value['key']);
                 $labelValues = $parts[3];
                 $bucket = $parts[4];
@@ -236,7 +222,7 @@ private function collectHistograms()
             sort($labels);
             foreach ($labels as $labelValues) {
                 $acc = 0;
-                $decodedLabelValues = $this->decodeLabelValues($labelValues);
+                $decodedLabelValues = json_decode($labelValues);
                 foreach ($data['buckets'] as $bucket) {
                     $bucket = (string) $bucket;
                     if (!isset($histogramBuckets[$labelValues][$bucket])) {
@@ -303,36 +289,4 @@ private function sortSamples(array &$samples)
             return strcmp(implode("", $a['labelValues']), implode("", $b['labelValues']));
         });
     }
-
-    /**
-     * @param array $values
-     * @return string
-     * @throws RuntimeException
-     */
-    private function encodeLabelValues(array $values)
-    {
-        $json = json_encode($values);
-        if (false === $json) {
-            throw new RuntimeException(json_last_error_msg());
-        }
-        return base64_encode($json);
-    }
-
-    /**
-     * @param string $values
-     * @return array
-     * @throws RuntimeException
-     */
-    private function decodeLabelValues($values)
-    {
-        $json = base64_decode($values, true);
-        if (false === $json) {
-            throw new RuntimeException('Cannot base64 decode label values');
-        }
-        $decodedValues = json_decode($json, true);
-        if (false === $decodedValues) {
-            throw new RuntimeException(json_last_error_msg());
-        }
-        return $decodedValues;
-    }
 }
diff --git a/src/Prometheus/Storage/APCU.php b/src/Prometheus/Storage/APCU.php
new file mode 100644
index 0000000..db45610
--- /dev/null
+++ b/src/Prometheus/Storage/APCU.php
@@ -0,0 +1,305 @@
+<?php
+namespace Prometheus\Storage;
+use Prometheus\MetricFamilySamples;
+use RuntimeException;
+class APCU implements Adapter
+{
+    const PROMETHEUS_PREFIX = 'prom';
+    /**
+     * @return MetricFamilySamples[]
+     */
+    public function collect()
+    {
+        $metrics = $this->collectHistograms();
+        $metrics = array_merge($metrics, $this->collectGauges());
+        $metrics = array_merge($metrics, $this->collectCounters());
+        return $metrics;
+    }
+    public function updateHistogram(array $data)
+    {
+        // Initialize the sum
+        $sumKey = $this->histogramBucketValueKey($data, 'sum');
+        $new = apcu_add($sumKey, $this->toInteger(0));
+        // If sum does not exist, assume a new histogram and store the metadata
+        if ($new) {
+            apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+        }
+        // Atomically increment the sum
+        // Taken from https://github.com/prometheus/client_golang/blob/66058aac3a83021948e5fb12f1f408ff556b9037/prometheus/value.go#L91
+        $done = false;
+        while (!$done) {
+            $old = apcu_fetch($sumKey);
+            $done = apcu_cas($sumKey, $old, $this->toInteger($this->fromInteger($old) + $data['value']));
+        }
+        // Figure out in which bucket the observation belongs
+        $bucketToIncrease = '+Inf';
+        foreach ($data['buckets'] as $bucket) {
+            if ($data['value'] <= $bucket) {
+                $bucketToIncrease = $bucket;
+                break;
+            }
+        }
+        // Initialize and increment the bucket
+        apcu_add($this->histogramBucketValueKey($data, $bucketToIncrease), 0);
+        apcu_inc($this->histogramBucketValueKey($data, $bucketToIncrease));
+    }
+    public function updateGauge(array $data)
+    {
+        $valueKey = $this->valueKey($data);
+        if ($data['command'] == Adapter::COMMAND_SET) {
+            apcu_store($valueKey, $this->toInteger($data['value']));
+            apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+        } else {
+            $new = apcu_add($valueKey, $this->toInteger(0));
+            if ($new) {
+                apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+            }
+            // Taken from https://github.com/prometheus/client_golang/blob/66058aac3a83021948e5fb12f1f408ff556b9037/prometheus/value.go#L91
+            $done = false;
+            while (!$done) {
+                $old = apcu_fetch($valueKey);
+                $done = apcu_cas($valueKey, $old, $this->toInteger($this->fromInteger($old) + $data['value']));
+            }
+        }
+    }
+    public function updateCounter(array $data)
+    {
+        $new = apcu_add($this->valueKey($data), 0);
+        if ($new) {
+            apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+        }
+        apcu_inc($this->valueKey($data), $data['value']);
+    }
+    public function flushAPCU()
+    {
+        apcu_clear_cache();
+    }
+    /**
+     * @param array $data
+     * @return string
+     */
+    private function metaKey(array $data)
+    {
+        return implode(':', array(self::PROMETHEUS_PREFIX, $data['type'], $data['name'], 'meta'));
+    }
+    /**
+     * @param array $data
+     * @return string
+     */
+    private function valueKey(array $data)
+    {
+        return implode(':', array(
+            self::PROMETHEUS_PREFIX,
+            $data['type'],
+            $data['name'],
+            $this->encodeLabelValues($data['labelValues']),
+            'value'
+        ));
+    }
+    /**
+     * @param array $data
+     * @return string
+     */
+    private function histogramBucketValueKey(array $data, $bucket)
+    {
+        return implode(':', array(
+            self::PROMETHEUS_PREFIX,
+            $data['type'],
+            $data['name'],
+            $this->encodeLabelValues($data['labelValues']),
+            $bucket,
+            'value'
+        ));
+    }
+    /**
+     * @param array $data
+     * @return array
+     */
+    private function metaData(array $data)
+    {
+        $metricsMetaData = $data;
+        unset($metricsMetaData['value']);
+        unset($metricsMetaData['command']);
+        unset($metricsMetaData['labelValues']);
+        return $metricsMetaData;
+    }
+    /**
+     * @return array
+     */
+    private function collectCounters()
+    {
+        $counters = array();
+        foreach (new \APCUIterator('/^prom:counter:.*:meta/') as $counter) {
+            $metaData = json_decode($counter['value'], true);
+            $data = array(
+                'name' => $metaData['name'],
+                'help' => $metaData['help'],
+                'type' => $metaData['type'],
+                'labelNames' => $metaData['labelNames'],
+            );
+            foreach (new \APCUIterator('/^prom:counter:' . $metaData['name'] . ':.*:value/') as $value) {
+                $parts = explode(':', $value['key']);
+                $labelValues = $parts[3];
+                $data['samples'][] = array(
+                    'name' => $metaData['name'],
+                    'labelNames' => array(),
+                    'labelValues' => $this->decodeLabelValues($labelValues),
+                    'value' => $value['value']
+                );
+            }
+            $this->sortSamples($data['samples']);
+            $counters[] = new MetricFamilySamples($data);
+        }
+        return $counters;
+    }
+    /**
+     * @return array
+     */
+    private function collectGauges()
+    {
+        $gauges = array();
+        foreach (new \APCUIterator('/^prom:gauge:.*:meta/') as $gauge) {
+            $metaData = json_decode($gauge['value'], true);
+            $data = array(
+                'name' => $metaData['name'],
+                'help' => $metaData['help'],
+                'type' => $metaData['type'],
+                'labelNames' => $metaData['labelNames'],
+            );
+            foreach (new \APCUIterator('/^prom:gauge:' . $metaData['name'] . ':.*:value/') as $value) {
+                $parts = explode(':', $value['key']);
+                $labelValues = $parts[3];
+                $data['samples'][] = array(
+                    'name' => $metaData['name'],
+                    'labelNames' => array(),
+                    'labelValues' => $this->decodeLabelValues($labelValues),
+                    'value' => $this->fromInteger($value['value'])
+                );
+            }
+            $this->sortSamples($data['samples']);
+            $gauges[] = new MetricFamilySamples($data);
+        }
+        return $gauges;
+    }
+    /**
+     * @return array
+     */
+    private function collectHistograms()
+    {
+        $histograms = array();
+        foreach (new \APCUIterator('/^prom:histogram:.*:meta/') as $histogram) {
+            $metaData = json_decode($histogram['value'], true);
+            $data = array(
+                'name' => $metaData['name'],
+                'help' => $metaData['help'],
+                'type' => $metaData['type'],
+                'labelNames' => $metaData['labelNames'],
+                'buckets' => $metaData['buckets']
+            );
+            // Add the Inf bucket so we can compute it later on
+            $data['buckets'][] = '+Inf';
+            $histogramBuckets = array();
+            foreach (new \APCUIterator('/^prom:histogram:' . $metaData['name'] . ':.*:value/') as $value) {
+                $parts = explode(':', $value['key']);
+                $labelValues = $parts[3];
+                $bucket = $parts[4];
+                // Key by labelValues
+                $histogramBuckets[$labelValues][$bucket] = $value['value'];
+            }
+            // Compute all buckets
+            $labels = array_keys($histogramBuckets);
+            sort($labels);
+            foreach ($labels as $labelValues) {
+                $acc = 0;
+                $decodedLabelValues = $this->decodeLabelValues($labelValues);
+                foreach ($data['buckets'] as $bucket) {
+                    $bucket = (string) $bucket;
+                    if (!isset($histogramBuckets[$labelValues][$bucket])) {
+                        $data['samples'][] = array(
+                            'name' => $metaData['name'] . '_bucket',
+                            'labelNames' => array('le'),
+                            'labelValues' => array_merge($decodedLabelValues, array($bucket)),
+                            'value' => $acc
+                        );
+                    } else {
+                        $acc += $histogramBuckets[$labelValues][$bucket];
+                        $data['samples'][] = array(
+                            'name' => $metaData['name'] . '_' . 'bucket',
+                            'labelNames' => array('le'),
+                            'labelValues' => array_merge($decodedLabelValues, array($bucket)),
+                            'value' => $acc
+                        );
+                    }
+                }
+                // Add the count
+                $data['samples'][] = array(
+                    'name' => $metaData['name'] . '_count',
+                    'labelNames' => array(),
+                    'labelValues' => $decodedLabelValues,
+                    'value' => $acc
+                );
+                // Add the sum
+                $data['samples'][] = array(
+                    'name' => $metaData['name'] . '_sum',
+                    'labelNames' => array(),
+                    'labelValues' => $decodedLabelValues,
+                    'value' => $this->fromInteger($histogramBuckets[$labelValues]['sum'])
+                );
+            }
+            $histograms[] = new MetricFamilySamples($data);
+        }
+        return $histograms;
+    }
+    /**
+     * @param mixed $val
+     * @return int
+     */
+    private function toInteger($val)
+    {
+        return unpack('Q', pack('d', $val))[1];
+    }
+    /**
+     * @param mixed $val
+     * @return int
+     */
+    private function fromInteger($val)
+    {
+        return unpack('d', pack('Q', $val))[1];
+    }
+    private function sortSamples(array &$samples)
+    {
+        usort($samples, function($a, $b){
+            return strcmp(implode("", $a['labelValues']), implode("", $b['labelValues']));
+        });
+    }
+    /**
+     * @param array $values
+     * @return string
+     * @throws RuntimeException
+     */
+    private function encodeLabelValues(array $values)
+    {
+        $json = json_encode($values);
+        if (false === $json) {
+            throw new RuntimeException(json_last_error_msg());
+        }
+        return base64_encode($json);
+    }
+    /**
+     * @param string $values
+     * @return array
+     * @throws RuntimeException
+     */
+    private function decodeLabelValues($values)
+    {
+        $json = base64_decode($values, true);
+        if (false === $json) {
+            throw new RuntimeException('Cannot base64 decode label values');
+        }
+        $decodedValues = json_decode($json, true);
+        if (false === $decodedValues) {
+            throw new RuntimeException(json_last_error_msg());
+        }
+        return $decodedValues;
+    }
+}
\ No newline at end of file
diff --git a/tests/Test/Prometheus/APC/CollectorRegistryTest.php b/tests/Test/Prometheus/APCU/CollectorRegistryTest.php
similarity index 53%
rename from tests/Test/Prometheus/APC/CollectorRegistryTest.php
rename to tests/Test/Prometheus/APCU/CollectorRegistryTest.php
index 23f7b84..3e2f631 100644
--- a/tests/Test/Prometheus/APC/CollectorRegistryTest.php
+++ b/tests/Test/Prometheus/APCU/CollectorRegistryTest.php
@@ -1,20 +1,20 @@
 <?php
 
 
-namespace Test\Prometheus\APC;
+namespace Test\Prometheus\APCU;
 
-use Prometheus\Storage\APC;
+use Prometheus\Storage\APCU;
 use Test\Prometheus\AbstractCollectorRegistryTest;
 
 /**
- * @requires extension apc
+ * @requires extension apcu
  */
 class CollectorRegistryTest extends AbstractCollectorRegistryTest
 {
 
     public function configureAdapter()
     {
-        $this->adapter = new APC();
-        $this->adapter->flushAPC();
+        $this->adapter = new APCU();
+        $this->adapter->flushAPCU();
     }
 }
diff --git a/tests/Test/Prometheus/APC/CounterTest.php b/tests/Test/Prometheus/APCU/CounterTest.php
similarity index 58%
rename from tests/Test/Prometheus/APC/CounterTest.php
rename to tests/Test/Prometheus/APCU/CounterTest.php
index 5ca3b5d..6bc4ef0 100644
--- a/tests/Test/Prometheus/APC/CounterTest.php
+++ b/tests/Test/Prometheus/APCU/CounterTest.php
@@ -1,20 +1,20 @@
 <?php
 
 
-namespace Test\Prometheus\APC;
+namespace Test\Prometheus\APCU;
 
-use Prometheus\Storage\APC;
+use Prometheus\Storage\APCU;
 use Test\Prometheus\AbstractCounterTest;
 
 /**
  * See https://prometheus.io/docs/instrumenting/exposition_formats/
- * @requires extension apc
+ * @requires extension apcu
  */
 class CounterTest extends AbstractCounterTest
 {
     public function configureAdapter()
     {
-        $this->adapter = new APC();
-        $this->adapter->flushAPC();
+        $this->adapter = new APCU();
+        $this->adapter->flushAPCU();
     }
 }
diff --git a/tests/Test/Prometheus/APC/GaugeTest.php b/tests/Test/Prometheus/APCU/GaugeTest.php
similarity index 57%
rename from tests/Test/Prometheus/APC/GaugeTest.php
rename to tests/Test/Prometheus/APCU/GaugeTest.php
index f1b2c04..9572e9a 100644
--- a/tests/Test/Prometheus/APC/GaugeTest.php
+++ b/tests/Test/Prometheus/APCU/GaugeTest.php
@@ -1,20 +1,20 @@
 <?php
 
 
-namespace Test\Prometheus\APC;
+namespace Test\Prometheus\APCU;
 
-use Prometheus\Storage\APC;
+use Prometheus\Storage\APCU;
 use Test\Prometheus\AbstractGaugeTest;
 
 /**
  * See https://prometheus.io/docs/instrumenting/exposition_formats/
- * @requires extension apc
+ * @requires extension apcu
  */
 class GaugeTest extends AbstractGaugeTest
 {
     public function configureAdapter()
     {
-        $this->adapter = new APC();
-        $this->adapter->flushAPC();
+        $this->adapter = new APCU();
+        $this->adapter->flushAPCU();
     }
 }
diff --git a/tests/Test/Prometheus/APC/HistogramTest.php b/tests/Test/Prometheus/APCU/HistogramTest.php
similarity index 59%
rename from tests/Test/Prometheus/APC/HistogramTest.php
rename to tests/Test/Prometheus/APCU/HistogramTest.php
index 19d41eb..882de65 100644
--- a/tests/Test/Prometheus/APC/HistogramTest.php
+++ b/tests/Test/Prometheus/APCU/HistogramTest.php
@@ -1,21 +1,21 @@
 <?php
 
 
-namespace Test\Prometheus\APC;
+namespace Test\Prometheus\APCU;
 
-use Prometheus\Storage\APC;
+use Prometheus\Storage\APCU;
 use Test\Prometheus\AbstractHistogramTest;
 
 /**
  * See https://prometheus.io/docs/instrumenting/exposition_formats/
- * @requires extension apc
+ * @requires extension apcu
  */
 class HistogramTest extends AbstractHistogramTest
 {
     public function configureAdapter()
     {
-        $this->adapter = new APC();
-        $this->adapter->flushAPC();
+        $this->adapter = new APCU();
+        $this->adapter->flushAPCU();
     }
 }