Err...
if i understood correctly you have a select down box and once the value is changed by the user the form should get submitted automatically using PHP.
When the select box is changed, it is still "within" the page. PHP runs on the server side, so unless the form is submitted, how would PHP know what has happened to the select box?
If you don't want to use javascript just for compatability with javascript disabled or older browsers, you could simply detect the browser and then show/not show the submit bottun!