How can I redirect in PHP with this setup below without getting header output errors, I understand that nothing can be printed to the browser before a header is set, I am lookin
As long as you have no script output before the header() function you should be fine. Check there are no echo's or whitespace. Also putting ob_start() at the beginning can help. sometimes there is invisible whitespace - changing the format of your document to ANSI or Unicode may help!
As a note (although I think you already know) header does not terminate the script so the exit() (which you have) is a definite requirement.
You need to buffer the ouput so that the HTTP header is not send on the first output. You can either buffer any ouput implicitly by enabling ouput_buffering or explicitly by calling ob_start. But the latter has to be called before the first output, so ideally in the first line of the script that’s initially called.
use { echo '<META HTTP-EQUIV="Refresh" Content="0; URL=process.php">';}
As already mentioned by the others use ob_start() or the output_buffer-setting to buffer the output. Apart from that it's from my point of view not a good practice to output content in the middle of functional code but this is a another topic.
You can find more information at Google or in this Article about Output Buffering in PHP.
Does the footer.inc.php and SOME-FILE-HERE.php write to the response stream immediately? Because if so, this won't work as you will have already written something before you sent the headers.
Can't you just do this:
<?php
validlogin($url); // call the function here
include ('header.inc.php');
include ('SOME-FILE-HERE.php');
include ('footer.inc.php');
?>
Or, put the include files in every one of the "SOME-FILE-HERE"-type files, if that's possible, so you end up with:
<?php
validlogin($url); // call the function here
include ('header.inc.php');
?>
<h1>Page heading</h1>
...page content etc...
<?php
include ('footer.inc.php');
?>