Решение:
function array_filter_keys(array $array, array $keys){
return array_intersect_key($array, array_flip($keys));
}
$input = array(
'key1' => 'qqq',
'key2' => 'www',
'key3' => 'eee'
);
var_dump(array_filter_keys($input, array('key1', 'key2', 'key5')));
array
'key1' => string 'qqq' (length=3)
'key2' => string 'www' (length=3)