$ sudo apt-get install php5 php5-apache2
$ sudo /sbin/service apache2 restart
$ sudo leafpad /var/www/html/test.php

<?php
phpinfo();
?>

http://localhost/test.php で表示されればOK

$ sudo chmod 777 /var/www/html

/et/apache2/conf/httpd.conf
/etc/apache2/conf.d/php5.conf
/etc/php5/php.ini