This is a complex question so I'll try and be precise.
I've experimented with AJAX but this form requires many changes to do it. I'd like to know if there are other options.
I have a form which takes a very long time to complete. Let's say the form exists on page A. The form submits and calls page B.
Page B looks like this, and gives the incorrect result (the page appears blank while loading):
<html>header</html>
<?php
longformresult()
?>
If I setup the page like this I get the result I am looking for (the page waits until the long function is done to load the page, allowing me to display a loading screen on page A).
<?php
longformresult()
?>
<html>header</html>
however this is where things get really tricky, the longformresult() can fail and break everything below it.
Is there some way I can tell php to wait until the page is fully loaded before sending the page, allowing me to have a loading screen on page A? Or prevent a die() from killing my page? I essentially need to delay the appearance of Page B, and actually just a sleep() is probably the best approach I've had so far:
<?php
sleep(4)
?>
<html>header</html>
<?php
longformresult()
?>
Edit: Conclusion (Not the fix I was looking for though)
I used ajax and removed 'Page B' entirely.
PHP waits by default to complete the function call before proceeding (see this previous answer), so PHP necessarily waits for the page to be fully loaded before returning.
AJAX would not require "many changes" if this is truly the behavior of the program. You would simply have a new file like C.php which returns the output of longformresult(), which you can later inject into your page.