PHP is available on Supporter accounts only. The currently installed version of PHP is 5.3.x with Suhosin patches for added security, and xcache for performance. PHP is run via mod_fcgid and suEXEC. This allows for good performance will still maintaining a high level of security, since all scripts are executed with the permissions of the owner of the script.
Suhosin is enabled to increase security, and because of this all your .php files must not be writable by "group" or "others". (Group or Others doesn't even need read permissions, so chmod 600 is fine for .php files. Static content will need read permission by Others however, like chmod 644 for example.)