How can I use phpinfo()?

For security reasons the phpinfo() function is disabled by default. If needed you can enable it like this:

Open /var/www/fcgi/your-domain.tld/php5/php.ini (according to what you use for the domain in question it can be also the php4 instead of the php5 directory)

Search for disable_functions and delete phpinfo in this line, so that it looks like this:

disable_functions = show_source, system, shell_exec,
passthru, exec, shell, symlink, popen, proc_open

After this restart your Apache to load the new config.

 
 # /etc/init.d/apache restart
 

Apply this change only when really needed.