To be clear, the phrase "pcntl_signal() doesn't stack the signal handlers, but replaces them. " means that you can still have different functions for different signals but only one function per signal.
In other words, this will work as expected
<?php
pcntl_async_signals(true);
pcntl_signal(SIGUSR1,function(){echo "received SIGUSR1";});
pcntl_signal(SIGUSR2,function(){echo "received SIGUSR2";});
posix_kill(posix_getpid(),SIGUSR1);
posix_kill(posix_getpid(),SIGUSR2);
// returns "received SIGUSR1" and then "received SIGUSR2"
?>