<?
$arr1 = Array('fn', 'ln', 'em');
$arr2 = Array('fn'=>'xyz',  'ano' => 'abc', 'ln'=>'122', 'em' => 'a@b.com', 'db'=>'xy');
$result = array_intersect_key($arr2, array_flip($arr1));

print_r($result);

Array
(
    [fn] => xyz
    [ln] => 122
    [em] => a@b.com
)