Hash password securely with sha512, salt and a secret key

hash.php

<?php
// Hash file using sha512, salt and a secret key.
// By github.com/lesander

// DON'T FORGET TO EDIT YOUR $secretkey !!!

// - Hashing your Password -
// Example 1 Request:
// http://domain.com/hash.php?s=myPassword
// Example 1 Result:
// {
//  ["salt"]="76d8e4fcbab6e1947cd2c97dcaec1f9d9416147351698a118b22e85029307655e44ffdd4a17a203b76b8e65d90fbe51f842c46559601310230125d24369c26c6",
//  ["hash"]="0b40fab1223bdda2d955dfcbb0eb35159c3a53f4cee1de5d78071125c0b997847870ad1869a825599c556c64a8f7baedb6287c2f6c37def5989ed84a2f82b409"
// }

// - Checking your Password -
// Example 2 Request:
// http://domain.com/hash.php?s=givenPassword&n=76d8e4fcbab6e1947cd2c97dcaec1f9d9416147351698a118b22e85029307655e44ffdd4a17a203b76b8e65d90fbe51f842c46559601310230125d24369c26c6
// Example 2 Result:
// {
//  ["salt"]="76d8e4fcbab6e1947cd2c97dcaec1f9d9416147351698a118b22e85029307655e44ffdd4a17a203b76b8e65d90fbe51f842c46559601310230125d24369c26c6",
//  ["hash"]="0b40fab1223bdda2d955dfcbb0eb35159c3a53f4cee1de5d78071125c0b997847870ad1869a825599c556c64a8f7baedb6287c2f6c37def5989ed84a2f82b409"
// }

$string = $_GET['s'];
if(empty($string)){
  die("");
}
$secretkey = "ENTER_SOME_RANDOM_STUFF_HERE";
if(empty($_GET['n'])){
  $salt = hash('sha512', microtime(true).mt_rand(10000,90000));
}
else{
  $salt = $_GET['n'];
}

$hash = hash('sha512', $salt.$string.$secretkey);
echo('{ ["salt"]="'.$salt.'", ["hash"]="'.$hash.'" }');
?>