Fibers for concurrency in PHP - Step-by-Step Execution

Concept Flow - Fibers for concurrency

Start main program

↓

Create Fiber

↓

Start Fiber

↓

Fiber runs until yield

↓

Main resumes, does work

↓

Resume Fiber

↓

Fiber continues or ends

↓

Main finishes

The main program creates and starts a fiber, which runs until it yields control back. The main program can then do other work and resume the fiber later, enabling concurrency.

Execution Sample

PHP

<?php
$fiber = new Fiber(function() {
    echo "Fiber started\n";
    Fiber::suspend();
    echo "Fiber resumed\n";
});
$fiber->start();
echo "Main after start\n";
$fiber->resume();

This code creates a fiber that prints a message, suspends itself, then prints another message when resumed. The main program starts the fiber, prints a message, then resumes the fiber.

Execution Table

Step	Action	Output	Fiber State	Main State
1	Create Fiber		Created	Ready
2	Start Fiber	Fiber started	Suspended (yielded)	Running
3	Main prints	Main after start	Suspended (yielded)	Running
4	Resume Fiber	Fiber resumed	Terminated	Running
5	End		Terminated	Finished

💡 Fiber terminates after finishing its function; main program ends after resuming fiber.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
$fiber state	Not created	Suspended (yielded)	Terminated	Terminated

Key Moments - 2 Insights

Why does the fiber stop after the first echo and not run the second echo immediately?

Because at step 2 in the execution_table, the fiber calls Fiber::suspend(), which pauses the fiber and returns control to the main program before the second echo.

What happens when the main program calls resume() on the fiber?

At step 4, resume() continues the fiber from where it was suspended, so it prints "Fiber resumed" and then the fiber ends.

Visual Quiz - 3 Questions

Look at the execution_table, what is the fiber state after step 2?

A Created

B Suspended (yielded)

C Terminated

D Running

💡 Hint

Check the 'Fiber State' column in row for step 2.

At which step does the main program print its message?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Output' columns in the execution_table.

If Fiber::suspend() was removed, what would happen to the fiber state after start()?

A Fiber would be terminated immediately

B Fiber would be created but not started

C Fiber would be suspended

D Fiber would throw an error

💡 Hint

Without suspend, fiber runs to end on start(), see how suspend affects state in execution_table step 2.

Concept Snapshot

Fibers in PHP allow pausing and resuming code blocks.
Create a Fiber with new Fiber(function() {...}).
Start it with start(), which runs until suspend().
Use suspend() to pause and resume() to continue.
This enables cooperative concurrency without threads.

Full Transcript

This example shows how PHP fibers work for concurrency. The main program creates a fiber that prints "Fiber started" and then suspends itself. The main program continues and prints "Main after start". Later, the main program resumes the fiber, which prints "Fiber resumed" and then ends. The fiber's state changes from created to suspended to terminated as it runs. This allows the main program and fiber to cooperate by yielding control back and forth.