Fix deprecated $php_errormsg in PHP 7.2

From the PHP 8.0 migration guide (https://www.php.net/manual/de/migration80.incompatible.php):
    The track_errors ini directive has been removed.
    This means that php_errormsg is no longer available.
    The error_get_last() function may be used instead.

Also the documentation on $php_errormsg (https://www.php.net/manual/en/reserved.variables.phperrormsg.php) says:
    This feature has been DEPRECATED as of PHP 7.2.0.
    Relying on this feature is highly discouraged.
    Use error_get_last() instead.

This commit breaks compatibility with PHP 5.6

Signed-off-by: Daniel Ziegenberg <daniel@ziegenberg.at>

Author: ziegenberg

lib/Horde/String.php

@@ -145,12 +145,9 @@ protected static function _convertCharset($input, $from, $to)
 
         /* Try iconv with transliteration. */
         if (Horde_Util::extensionExists('iconv')) {
-            unset($php_errormsg);
-            ini_set('track_errors', 1);
+            error_clear_last();
             $out = @iconv($from, $to . '//TRANSLIT', $input);
-            $errmsg = isset($php_errormsg);
-            ini_restore('track_errors');
-            if (!$errmsg && $out !== false) {
+            if (is_null(error_get_last()) && $out !== false) {
                 return $out;
             }
         }
@@ -484,18 +481,15 @@ protected static function _pos(
     )
     {
         if (Horde_Util::extensionExists('mbstring')) {
-            unset($php_errormsg);
-            $track_errors = ini_set('track_errors', 1);
+            error_clear_last();
             $ret = @call_user_func('mb_' . $func, $haystack, $needle, $offset, self::_mbstringCharset($charset));
-            ini_set('track_errors', $track_errors);
-            if (!isset($php_errormsg)) {
+            if (is_null(error_get_last())) {
                 return $ret;
             }
         }
 
         if (Horde_Util::extensionExists('intl')) {
-            unset($php_errormsg);
-            $track_errors = ini_set('track_errors', 1);
+            error_clear_last();
             $ret = self::convertCharset(
                 @call_user_func(
                     'grapheme_' . $func,
@@ -506,8 +500,7 @@ protected static function _pos(
                 'UTF-8',
                 $charset
             );
-            ini_set('track_errors', $track_errors);
-            if (!isset($php_errormsg)) {
+            if (is_null(error_get_last())) {
                 return $ret;
             }
         }