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.
Multiple shell programs on various Unix systems, including (1) tcsh, (2) csh, (3) sh, and (4) bash, follow symlinks when processing << redirects (aka here-documents or in-here documents), which allows local users to overwrite files of other users via a symlink attack.