$ which php /usr/bin/php
$ ls -l /usr/bin/php -rwxr-xr-x 1 root root 2849248 Jul 16 2008 /usr/bin/php
Your system administrator will know where PHP is installed:
$ which php /usr/bin/php
By default anyone on the system can use it to execute PHP scripts:
$ ls -l /usr/bin/php -rwxr-xr-x 1 root root 2849248 Jul 16 2008 /usr/bin/php