Since php >= 5.3 handles Closures, you are now able to define the Callback directly.
Try this:
<?php
declare(ticks = 1);
pcntl_signal(SIGUSR1, function ($signal) {
echo 'HANDLE SIGNAL ' . $signal . PHP_EOL;
});
posix_kill(posix_getpid(), SIGUSR1);
die;
?>