include ("foo.php") not working

I have a strange problem which I cannot seem to resolve. I am running PHP 4.0.6 on my Mac OSX using apache and MySQL. Everything works fine except for one problem. Like many developers I make extensive use of the include funtion but it does not seem to work correctly on this installation. when I include the statement <? include ("foo.php"); ?> then rather than parsing the file and displaying the results the whole file is just dumped to screen as a text file. Interesting the same problem occurs whatever the file extension so the same thing happens for foo.htm, foo.html and foo.txt! I do not get this problem with my SUSE Linux 7.1 box which is also running PHP, Apache and MySQL. I did compile and make the PHP from the source. Your help and advice would be much appreciated. TIA Jon