To learn the offset or array key for a specific array element in a given array turns out to be fairly easy, thanks to the extra search parameter that the always useful array_keys (return all the keys or a subset of the keys of an array) function offers us.
From the manual description: array array_keys ( array $input [, mixed $search_value = NULL [, bool $strict = false ]] ) – array_keys() returns the keys, numeric and string, from the input array. If the optional search_value is specified, then only the keys for that value are returned. Otherwise, all the keys from the input are returned.
Knowing this, we can now find exactly where a specific element sits by simply entering a search value when calling the array_keys function, which in practice would look something like this:
$array = array("blue", "red", "green", "blue", "blue"); print_r(array_keys($array, "blue")); /* Array (  => 0  => 3  => 4 ) */
The above example shows us that the array element “blue” appears at offset 0, 3 and 4 in the source array.
Related Link: http://www.php.net/manual/en/function.array-keys.php