Bitwise operation giving wrong results

["sean greenslade" <zootboysean@xxxxxxxxx>]

I have the following code as a test:
<?php
$a = $_GET['a'];
$b = $_GET['b'];
echo  $a . " & " . $b . " = ";
$out = $a & $b;
echo $out;
//echo 15 & 2;
?>

if I set a to 15 and b to 2 in the URL like so:

test.php?a=15&b=2

it outputs zero as the answer. When I run the hard-coded '&' operation (the
commented out echo), it returns 2. I'm stumped.
-- 
--Zootboy