What happens behind the PHP pcntl_fork()?

I have spend hours, days, weeks trying to understand how PHP pcntl really works and still dont know exactly what is really happening.

Why parents and childs ? I am using Debian (am far for being an expert in computers I must admit) but all I want to do is receive a message from my webSocket client (which I have already managed to do) and keep listening other users.

In other words, if I receive a message and use a sleep(5); then the server will not listen any other messages until this piece of code is finished.

Note that the sleep(5) is for test purposes. I plan to process hundreds of messages simultaneously and thats why am trying to figure out how could pcntl be helpfull.

$pid = pcntl_fork();

echo "start\n";

if($pid) {
  // parent process runs what is here
  echo "parent\n";
}
else {
  // child process runs what is here
  echo "child\n";
}

echo "end\n";

What exactly is happening behind the above script ? Why am I getting this result ?

start

parent

end

start

child

end

Could someone please explain me what is the computer "thinking" ?

Solution

This is basically what happens:

                    pcntl_fork()
                       ||
      +----------------++------------------+
    parent                               child

    "start"                              ...
    "parent"                             ...
    "end"                                ...
                                         "start"
                                         "child"
                                         "end"

Any code after pcntl_fork() gets executed by both parent and child process; you can think of the child as a clone of the parent, except that the outcome of pcntl_fork() is 0; it's like the code enters a different reality, not unlike Back to the future ;-)

This is just one of the possible outcomes, though; depending on cpu usage, the child's "start" may occur together with the parent's "parent" output.