Determine which are user defined keys?

Given the following array: <?php $myArr = array( 'joe' => 'bob', "0" => 'briggs', 'whatever', 'whereever'); echo '<pre>' . print_r( $myArr, TRUE ) . '</pre>'; ?> Array ( [joe] => bob [0] => briggs [1] => whatever [2] => whereever ) "joe" and "0" are keys that I created whereas the key "1" and "2" are keys assigned by PHP when the array was created. When iterating through an array, is there a way to determine which were generated by PHP? I can't rely on whether or not the key is an integer because it's quite possible that such a key was user generated. I've gone through the docs and based on what I've read, I don't think something like this is possible but I'm hoping that's not the case. Any pointers? thnx, Christoph