<?php 
$a = exec("./a.sh");
var_dump($a);

#!/bin/bash
echo "11"
(sleep 10) &

#!/bin/bash
echo "11"
(sleep 10 > /dev/null) &

Note:

If a program is started with this function, in order for it to continue running in the background, the output of the program must be redirected to a file or another output stream. Failing to do so will cause PHP to hang until the execution of the program ends.