(PHP 4 >= 4.0.5)

array_search --  Searches the array for a given value and returns the corresponding key if successful


mixed array_search (mixed needle, array haystack [, bool strict])

Searches haystack for needle and returns the key if it is found in the array, FALSE otherwise.

If the optional third parameter strict is set to TRUE then the array_search() will also check the types of the needle in the haystack.

See also in_array().