Sorry, the test script is incorrect. It should be:

pcntl_signal(SIGCHLD, SIG_IGN);
$pid = @pcntl_fork();
if ($pid == -1)
{
  exit(pcntl_get_last_error() . ': ' . pcntl_strerror(pcntl_get_last_error()));
}
elseif ($pid == 0)
{
  exit();
}

sleep(60);
exit("Done");

This also creates a <defunct> process when pcntl_signal(SIGCHLD, SIG_IGN) should have removed it.

This bug exists because ZEND_SIGNALS was defaulted to "yes" with PHP 7.1.  The problem with this is that sleep() will sleep for the number of seconds, OR, is interrupted by a signal that is not ignored.

With your test script with pcntl_signal(SIGCHLD, SIG_IGN), you're instructing PHP to ignore the end of a child.  However, ZEND_SIGNALS, zend_sigaction() sets the set handler into the SIGG(handlers), but, will set the actual signal handler to zend_signal_handler_defer, which is not SIG_IGN.  As such, during the sleeping, when a SIGCHLD is raised, sleep bails because it is not "ignored" at the system level, and ends up calling zend_signal_handler_defer which will not process the signal.

I believe a safe fix would be to in zend_sigaction, only set the defer handler IF the incoming sigact's handler is not SIG_IGN.  Will make a PR and see if it's acceptable or not.

Would this also fix the issue where pcntl_signal(SIGCHLD, SIG_IGN) no longer stops <defunct> threads from remaining until the main process has ended? In 7.0 threads would exit cleanly, but since upgrading to 7.1 they all sit as <defunct> now.

I would assume as much, I wasn't directly able to replicate the <defunct> process. I tried on both my Debian setup & MacOS 10.12, which both lack the <defunct> but do both sleep correctly with the fix in place.
The output I get with PHP 7.1, and my fix are as follows:

dwalker@linux:~/src/php PHP-7.1 › time sapi/cli/php e.php && ps aux | grep defunc
Parent
Child Dying
After Sleep
real    0m0.022s
user    0m0.004s
sys     0m0.016s
dwalker  24037  0.0  0.0  12784   940 pts/3    S+   08:38   0:00 grep --color=auto defunc



dwalker@linux:~/src/php fix-73783 › time sapi/cli/php e.php && ps aux | grep defunc
Parent
Child Dying
After Sleep

real    0m10.015s
user    0m0.012s
sys     0m0.004s
dwalker  16683  0.0  0.0  12784   900 pts/3    S+   08:37   0:00 grep --color=auto defunc