Some PHP functions are disabled by default in concern of your website security.
The list of the disabled functions:
system, exec, shell_exec, passthru, mysql_list_dbs, ini_alter, dl, symlink, link, chgrp, leak, popen, escapeshellcmd, apache_child_terminate, virtual, mb_send_mail
You can enable or disable them on your Shared or Cloud Hosting plans by navigating to PHP configurations section.
Listed PHP functions are disabled by default. If you want to enable it, you need to delete it from the list. Don’t forget to press the Save button!
To reset functions to default, you can press Reset options button here:
NOTE: Here's an article, how to find Control Panel over here.