Some versions of Debian based distros (like Ubuntu) disable by default the usage of PHP in userdir websites (http://server/~user).

To enable, remove the following lines from /etc/apache2/mods-enabled/php5.conf:

<IfModule mod_userdir.c>
    <Directory /home/*/public_html>
        php_admin_value engine Off
    </Directory>
</IfModule>