is there a way to disable it?
Yes, you only need to use the JSON_UNESCAPED_SLASHES flag.
!important read before: https://stackoverflow.com/a/10210367/367456 (know what you're dealing with - know your enemy)
json_encode($str, JSON_UNESCAPED_SLASHES);
If you don't have PHP 5.4 at hand, pick one of the many existing functions and modify them to your needs, e.g. http://snippets.dzone.com/posts/show/7487 (archived copy).
<?php
/*
* Escaping the reverse-solidus character ("/", slash) is optional in JSON.
*
* This can be controlled with the JSON_UNESCAPED_SLASHES flag constant in PHP.
*
* @link http://stackoverflow.com/a/10210433/367456
*/
$url = 'http://www.example.com/';
echo json_encode($url), "\n";
echo json_encode($url, JSON_UNESCAPED_SLASHES), "\n";
Example Output:
"http:\/\/www.example.com\/"
"http://www.example.com/"