how to enable shell_exec and exec on php?
If you are not the root on the machine, and exec() function is disabled, then you can't enable it by yourself.
See http://php.net/manual/en/ini.core.php#ini.disable-functions
disable_functions string
This directive allows you to disable certain functions for security reasons. It takes on a comma-delimited list of function names. disable_functions is not affected by Safe Mode.
Only internal functions can be disabled using this directive. User-defined functions are unaffected.
This directive must be set in php.ini For example, you cannot set this in httpd.conf.