Errr - this should be set in the header of the web page before it gets dumped to the browser. If you say other browsers work when connecting to the same server then it's a bit confusin.
If you don't do anything then PHP will automagically use a text/html header so the browser knows to render html. If you are having problems try explicitly setting it to text/html like so :
header('Content-type: text/html');
This should go at the top of your file before ANY other output is done.
