<?php

/*
 * This function will return the keys of elements in the 
 * haystack where the value is found in array needle
 */

function array_value_intersect_keys( $array_haystack, $array_needle ){
    $intersected = array_intersect( $array_haystack, $array_needle );
    return array_keys( $intersected );
}

// usage 

$array_haystack = array( 1 => 2, 2 => 5, 'red' => 8, 9 => 14 );

$array_needle = array( 2, 8 );

$array_keys_of_intersecting_values = array_value_intersect_keys( $array_haystack, $array_needle );

print_r( $array_keys_of_intersecting_values );
?>

returns
Array
(
    [0] => 1
    [1] => red
)