Enable exec()

How can i enable php exec() ?

I looked for disable_functions=, but there is nothing is arranged for this i mean it is blank. like this : "disable-functions ="
For installation SiberianCMS is need enable
PHP extensions: GD, PDO MySQL, Simplexml, Curl, DOM, iconv, mbstring, mod_rewrite.
PHP functions: exec() , allow url_fopen
Zip
Unzip
OpenSSL 1.0.1 minimum
php-sqlite3 or glibc >= 2.12

Hello,

if you do not see the function in disabled_functions list, it does not disabled I guess.