just for the beginners... PHP is actually quite simple just keep in mind, that anything done by PHP is done on the webserver, never on the clientside.
Therefore you "visitor" request your form page from the webserver (e.g. form.php), the webserver executes all the php stuff and sends the result as a "normal" HTML document to your visitor. As long as the "visitor" does nothing, no php is executed. After the "visitor" submits the form, the specified page is requested (e.g. save_formdata.php) all the PHP in there gets executed (e.g. validate the form data, save the form data in a db etc.) and if you place the Header() call at the end of your php script, the specified page (thankyou.htm) gets called, without the user noticing that there was a page in between.
Hope that gave you a quick overview over the really basic meaning of Server Side ...
On the other hand: If you want to display a message: "Thanks for submitting" and want to change the page after displaying this message for e.g. 5 sec. You need JavaScript, remember a Webserver is only able to ANSWER Request, a Webserver can't send a document that wasn't requested by the client (Therefore PHP is pretty useless for this task). Therefore you need to emulate the client request via Java Script, the Metatag you mentioned pointed in the right direction... just some syntax errors, try:
echo "<META http-equiv=\"refresh\" content=\"5; URL=http://www.php.net\">\n";
hope that helped
and if you looked around and can't find an answer just ask, IMHO that's what a forum is designed for 🙂
Rob