I don't see any reference to PHP on my server, but I don't recall anything like that for quite a while, since the days when you could select which PHP version to use. Anyway, you might look in your .htaccess file (if Apache server) and see if you have anything that specifies a particular PHP version to use (AddHandler, etc.). Try commenting it out and just using the default PHP (probably 5.6 now). Is there anything else redirecting .php files to something else? The following .php script:
<?php phpinfo(); ?>will tell you what version you're running (if PHP is running by that point).
If you need to go back to an earlier PHP version, you'll have to talk to support about that -- maybe they'll move you to a server which hasn't been upgraded yet. It would be better to upgrade your software to work with PHP 5.6 (and 7.0 will be coming before too long).
Oh, by the way, a 500 error can indicate something other than a PHP problem. You may have something invalid in your .htaccess or php.ini file, such as phpflag, phpvalue, or options commands. Recent server upgrades may have caused some of those obsolete (or insecure) commands to now cause a 500 error.