Excessive use of array_key_exists()

Hello,

Drupal heavily relies on the php function array_key_exists(). This function is VERY slow compared to a simple "isset($array[$key])". See notes on http://php.net/array_key_exists
I really would recommend to change all occurences of array_key_exists($key, $array) to isset($array[$key]) to make Drupal even better by increasing its performance.

Thank you,
Alex