"Non-blocking" process in PHP

Question

a help.I have the following code.

$output = shell_exec("php /sites/empresa/php/faturamento/gerar_fat_simulado.php ".$cliente.' '.$mes.' '.$ano.' '.$tipo);

It runs through the shell a fairly time-consuming routine that blocks the entire system until it is completed. I know that PHP does not work with Threads. Is there any way to make it not block the rest of the system? Thank you.