double_hash.php

<?php
function double_hash($key1, $key2, $salt = null, $callback = "sha1") {
	if (!is_numeric($key1) || !is_numeric($key2)) {
		return false;
	}
	
	$key1 = (int)$key1;
	$key2 = (int)$key2;
	
	$first = null;
	$second = null;
	if ($key1 > $key2) {
		$first = $key1;
		$second = $key2;
	} else {
		$first = $key2;
		$second = $key1;
	}
	
	$str = $first."|".$second.'|'.$salt;
	
	if (is_string($callback) || is_array($callback)) {
		return call_user_func($callback, $str);
	} elseif (is_object($callback) && $callback instanceof \Closure) {
		return $callback($str);
	}
	return false;
}

var_dump(double_hash(123, 456,"abc"));
var_dump(double_hash(123, 456,"abc"));
var_dump(double_hash(12, 3456,"abc"));
var_dump(double_hash(1234, 56,"abc"));

var_dump(double_hash(1234, 56,"abc",function($str){
	return $str;
}));