I assume you want to get the URL. I would use regex to get the url out of it. The regex would be http[\S]*.mp4.
In code this is:
$string = '[video width="640" height="360" mp4="http://click.ir/wp-content/uploads/2017/09/Spinning_cylinder_shaped_elevator.mp4"][/video]"';
// use regex to fetch the url
preg_match("/http[\S]*\.mp4/", $string, $matches);
// get first match
$url = $matches[0];
// $url is http://click.ir/wp-content/uploads/2017/09/Spinning_cylinder_shaped_elevator.mp4
There are bether and nicer regex for URL, but in your case this would work. Depending on your Incoming data, you can adjust your regex to get what you want.
See preg_match docs: http://php.net/manual/en/function.preg-match.php
Online Regex tester: https://www.regexpal.com/
Lots of Regex infos: http://www.regular-expressions.info/