weird problem with index page

Hi, I am not sure if this is a PHP or Apache problem, but I am hoping someone can tellme what is wrong. When I go to www.domain.com on my server it shows the page as text instead of parsing the php code. If I go to www.domain.com/index.php (same page) it runs as it should. if I go to www.domain.com/subdir it sya s page not found even though there is a index.php file in that directory. I am at a loss as to what is wrong. index.php is listed inthe directoryindex for apache so I dont understand why it is doing all this. If anyone can help me it would be appreciated. Thanks Loll