Ok, This isn't a valid PHP Function, It's an alias off Array_key_exist, So this is one off them cases where everyone was actually right, Spud was right the script will run, However we was right as it isn't a PHP Function, And I've read on two site's that it isn't going to be kept as an alias, Infact some people have issue's with it on PHP5.
Sorry Spud, However we was still kinda right lol