/**
  * Returns the substring of the given subject string.
  * @since           2.1.2
  */
 public static function getSubstring($sString, $iStart, $iLength = null, $sEncoding = null)
 {
     self::$_bFunctionExists_mb_substr = isset(self::$_bFunctionExists_mb_substr) ? self::$_bFunctionExists_mb_substr : function_exists('mb_substr') && function_exists('mb_internal_encoding');
     if (!self::$_bFunctionExists_mb_substr) {
         return substr($sString, $iStart, $iLength);
     }
     $sEncoding = isset($sEncoding) ? $sEncoding : mb_internal_encoding();
     return mb_substr($sString, $iStart, $iLength, $sEncoding);
 }