500 Error on index.php file

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http://askubuntu.com – I have found a question very similar to this, which I thought would solve my problem. However, after changing my .htaccess file, with index.php on the end of the list, and then index.php as the only entry, I still cannot get index.php to display. Every other page works fine, but I don't know how to fix this. Thanks. (HowTos)