The Apache module for PHP 4.0.0 through PHP 4.0.4, when disabled with the 'engine = off' option for a virtual host, may disable PHP for other virtual hosts, which could cause Apache to serve the source code of PHP scripts.
PHP Apache module 4.0.4 and earlier allows remote attackers to bypass .htaccess access restrictions via a malformed HTTP request on an unrestricted page that causes PHP to use those access controls on the next page that is requested.