Clarify when fibers can be switched

[Provoker]

Description

Since version 8.2.0 when trying to create fiber in a function passed to pcntl_signal an "Cannot switch fibers in current execution context" error occurs.
The documentation says only about limitation in the destructor.

The following code:

<?php

pcntl_signal(SIGINT, function () {
	$fiber = new Fiber(function () {
		Fiber::suspend('test');
	});

	var_dump($fiber->start());
});

posix_kill(getmypid(), SIGINT);
pcntl_signal_dispatch();

Resulted in this output:

string(4) "test"

But I expected this output instead:

Uncaught FiberError: Cannot switch fibers in current execution context

PHP Version

PHP 8.2.0

Operating System

No response

[iluuu1994]

This is intentional (php/php-src#9028), but the documentation could be improved.

[davidbuzz]

For those looking for more info, u can no longer use fibers from register_tick_function or pcntl_signal, so something like this worked in php8.1, but not moving forward...

<?php
...
register_tick_function('scheduler');
...
 function scheduler () {
    if(Fiber::getCurrent() === null) {
      return;
    }
    // running Fiber::suspend() in this if condition will prevent an infinite loop!
    if(count(self::$fibers) > 1)
    { 
        Fiber::suspend();
    }
  }