Many people use FASTCGI to serve up their PHP to the server.  One such version of this is called PHPFRM and when you enable this on the server it will automatically disable certain PHP functions (even if you have those enabled in your phpini file)  

For example:  in your php ini file you have the following:

disable_functions = show_source, system, passthru, popen, proc_open

if you notice that exec and shell_exec are not listed in the disabled_functions above. 

However, when you enable PHPFRM it will automatically disable exec, shell_exec and other functions (at this time i dont have a list of those)

For those of use that use FFMPEG we need the exec and shell_exec so what do you do?

Well you can disable PHPFRM and use something else...  Or  you can just make a small file on the server to tell PHPFRM what you need. 

The process is outlined here:   

https://forums.cpanel.net/threads/enabling-php-fpm-forces-disable_functions.590799/#post-2391379

NOTE:  you dont need to create the ApachePHPFPM directory its already there. I was in my version. 

Just be sure to make some notes in the file to remind yourself what the file is for.  

#note

#note

Also something very important in that conversation is 

Additionally, keep in mind the PHPINFO output on the website will match what you've configured in your custom PHP-FPM configuration file, despite the fact that additional PHP functions are disabled 

that means that even if you have other stuff disabled in your normal php ini file, the PHPINFO will show what PHPFRM has.  So when you are troubleshooting please remember that.  

dave :)