Merge pull request facebookarchive#548 from DragonBe/mb_substr

GitHub Issue facebookarchive#546: Received a fatal because mb_substr was not found.

Author: gfosco

src/Facebook/PseudoRandomString/PseudoRandomStringGeneratorTrait.php

@@ -48,11 +48,15 @@ public function validateLength($length)
      *
      * @param string $binaryData The binary data to convert to hex.
      * @param int    $length     The length of the string to return.
+     * @throws \RuntimeException Throws an exception when multibyte support is not enabled
      *
      * @return string
      */
     public function binToHex($binaryData, $length)
     {
-        return mb_substr(bin2hex($binaryData), 0, $length);
+        if (true !== extension_loaded('mbstring')) {
+            throw new \RuntimeException('Multibyte support required');
+        }
+        return \mb_substr(\bin2hex($binaryData), 0, $length);
     }
 }