Fatal Error – Not Running PHP 5.3

The PHP version number is returned by phpversion() native function (https://php.net/manual/en/function.phpversion.php). Make sure you have the right version enabled on your server. Also make sure that you don’t have two different versions or that you forgot to restart your server after upgrading.
Please check what this function return

Thanks

Add the following to .htaccess:

# Use PHP 5.3
AddType application/x-httpd-php53 .php

It works for me.

what does it mean ” .htaccess”, where to find this file or we have to creat this file, and where to put this file? thanks

You can find this file in the root of your site. It is created when you make changes to Settings > Permalinks, from Default to any other option.