diff --git a/src/CreditCard.php b/src/CreditCard.php
index 049c497..48fa7bf 100755
--- a/src/CreditCard.php
+++ b/src/CreditCard.php
@@ -223,12 +223,13 @@ protected static function luhnCheck($number)
     {
         $checksum = 0;
         for ($i = (2 - (strlen($number) % 2)); $i <= strlen($number); $i += 2) {
-            $checksum += (int)($number{$i - 1});
+            $checksum += (int) substr($number, $i - 1, 1);
         }
 
         // Analyze odd digits in even length strings or even digits in odd length strings.
         for ($i = (strlen($number) % 2) + 1; $i < strlen($number); $i += 2) {
-            $digit = (int)($number{$i - 1}) * 2;
+            $digit = (int) substr($number, $i - 1, 1);
+            $digit *= 2;
             if ($digit < 10) {
                 $checksum += $digit;
             } else {