PHP forked process not exit completely and become zombie

PHP forked process not exit completely and become zombie

This is a simplified program.

The test.php is a daemon program, always running there.
It forks process to work on some task.
Once finishing work, the forked process exits.
But when the forked process exits, it becomes zombie.

How to make the forked process exit completely without becoming zombie?

#!/usr/bin/php
<?php

while (1) {
  sleep(1);
 
  $pid = pcntl_fork();
  if (!$pid) {
    $mypid = getmypid();  
    sleep(5);
    print "pid=$mypid finish work \n";
    exit();
  }
 
  sleep(1);
} // while

?>

./test.php
... daemon running ...

$ ps -ef | grep mqp
ubuntu   10084 10073  0 12:21 pts/0    00:00:00 /usr/bin/php ./test.php
ubuntu   10085 10073  0 12:21 pts/0    00:00:00 /usr/bin/php ./test.php
ubuntu   10074 10073  0 12:21 pts/0    00:00:00 [test.php] <defunct>
ubuntu   10075 10073  0 12:21 pts/0    00:00:00 [test.php] <defunct>