I recently ran into a problem with my WordPress installations.  I could not auto update my plugins.  After some research, I found that it was a problem with the hosting server having php4 and php5 enabled at the same time.  This is common practice for shared hosting plans.   I found that adding the following lines to the .htaccess file will fix the problem.

AddType x-mapp-php5 .php
AddHandler x-mapp-php5 .php

Before doing this, I would recommend you backup your WordPress installation (incl. database, files and folders).