I have had this problem too.
In my case, it was due to incorrect addHandler, and addType directives in the .htaccess file of the site.
I was also parshing html files as php, so in my .htaccess file I had
Code:
AddType text/x-server-parsed-html .html .htm
AddHandler application/x-httpd-php .htm .html
This needed to be
Code:
AddType application/x-httpd-php5 .html .htm
AddHandler application/x-httpd-php5 .html .htm
The actual type depends on how apache/php was compiled.
Look in /usr/local/apache/conf/php.conf to see the correct format for your setup.
Tom