Should I write the form and processor in the same file or seperate them?

In terms of best practices of PHP, is it better practice to write the processing script and the form in the same file or have them separate in different files? How does each scale in proportion to each other for larger projects?

For example, should I write it as:

login.php

<?php
process_form_here();
?>
<form>
<input>
</form>

Or should I write it as:

processor.php

<?php
process_form_here();
?>

login.html

<form>
<input>
</form>

I've been trying to decide for a while between making it more abstract or more condensed... Is there a certain way that most web developers follow?