Merge branch 'hotfix/227'

Close #227

Author: weierophinney

src/CreditCard.php

@@ -279,7 +279,7 @@ public function isValid($value)
         $foundl = false;
         foreach ($types as $type) {
             foreach ($this->cardType[$type] as $prefix) {
-                if (substr($value, 0, strlen($prefix)) == $prefix) {
+                if (0 === strpos($value, $prefix)) {
                     $foundp = true;
                     if (in_array($length, $this->cardLength[$type])) {
                         $foundl = true;