Weird PHP error: exec() hangs sometimes on simple script - php

Question

Heyas,
So this simple exec() script runs fine for the first two times, in trying to generate a PDF file from a webpage (using wkhtmltopdf).
It first) deletes the existing file, and second) creates the new PDF file in its place. If I run the script a second time, it deletes the file again, and then creates a new one, as expected. However, if I run it one more time, it deletes the files, creates a new one, but then the script seems to hang until the 30-second 504 timeout error is given. The script, when it works, only takes about 3 seconds to run/return. It also kills the entire server (any other local PHP sites no longer work). If I restart the PHP server, everything still hangs (with no success). Interestingly, if I run the script once, and then restart the PHP server, I can keep doing this without issue (but only generating the PDF up to two times). No PHP errors are logged.
Why would it be stalling out subsequent times?
$filePath = 'C:\\wtserver\\tmp\\order_' . $orderId . '.pdf';
// delete an existing file
if(file_exists($filePath)) {
if(!unlink($filePath)) {
echo 'Error deleting existing file: ' . $filePath;
return;
}
}
// generates PDF file at C:\wtserver\tmp\order_ID.pdf
exec('wkhtmltopdf http://google.com ' . $filePath);
I've tried a simple loop to check for the script's completion (successful output), and then try to exit, but it still hangs:
while(true) {
if(file_exists($filePath)) {
echo 'exit';
exit(); // have also tried die()
break;
}
//todo: add time check/don't hang
}
If I can't figure this bit out, for now, is there a way to kill the exec script, wrapping it somehow? The PDF is still generated, so the script is working, but I need to kill it and return a response to the user.
Solution:
Have to redirect standard output AND standard error, to end the process immediately, ie. in Windows:
exec('wkhtmltopdf http://google.com ' . $filePath . ' > NUL 2> NUL');

Answer 1

do you know that you can run the executable in background, like this
exec($cmd . " > /dev/null &");
This way you can immediately come out of it.