PHP Array Search Return Multiple Keys

I'm trying to search an array and return multiple keys

<?php
$a=array("a"=>"1","b"=>"2","c"=>"2");
echo array_search("2",$a);
?>

With the code above it only returns b, how can I get I to return b and c?

As it says in the manual for array_search:

To return the keys for all matching values, use array_keys() with the optional search_value parameter instead.

Example:

$a=array("a"=>"1","b"=>"2","c"=>"2");
print_r(array_keys($a, "2"));

Result:

Array
(
    [0] => b
    [1] => c
)