Why does ignoring SIGCHLD cause exit code -1?

Question

This code expectedly outputs "0":

exec('date', $output, $resultCode);
echo $resultCode . PHP_EOL;

However, if I prepend this line:

pcntl_signal(SIGCHLD, SIG_IGN);

then the output is "-1".

Why?

Background: what I am trying to do is run exec in a forked process. The pcntl_signal line is required to avoid the exited children becoming zombies.