PHP is an open source and powerful server side scripting language, can run  in Windows, Linux, Mac and other operating system. Some very useful functions are becomes dangerous function because  hackers easily hacked websites in hosting servers..

If you want to disable PHP function add these function names in php.ini,

Most probably will get PHP configure file from  /etc/php.ini directory or  may also get in /usr/local/lib/php.ini,  If you could not find give the below command,

# whereis php
# whereis php.ini
php: /usr/bin/php /usr/lib64/php /etc/php.d /etc/php.ini /usr/include/php /usr/share/php /usr/share/man/man1/php.1.gz

For example disabled few fuctions,
# vim /usr/local/lib/php.ini

disable_functions =  "apache_setenv, escapeshellarg, escapeshellcmd, exec,  ftp_put, ftp_get,  inject_code, passthru, popen, proc_open, proc_terminate, symlinked, symlink,  show_source, shell_exec, system"