Yield keyword behavior in PHP - Step-by-Step Execution

Concept Flow - Yield keyword behavior

Function called

↓

Start execution

↓

Yield value

↓

Pause function

↓

Caller receives value

↓

Caller requests next

↓

Resume function

↓

Repeat yield or end

↓

Function ends

↓

Iteration stops

The function runs until it hits a yield, returns a value, pauses, then resumes when the caller asks for the next value, repeating until done.

Execution Sample

PHP

<?php
function countToThree() {
  yield 1;
  yield 2;
  yield 3;
}

foreach (countToThree() as $number) {
  echo $number . " ";
}

This code yields numbers 1, 2, and 3 one by one, printing them with spaces.

Execution Table

Step	Action	Yielded Value	Function State	Output
1	Function called, starts execution	N/A	At first yield
2	Yield 1	1	Paused after yield 1	1
3	Resume function	N/A	At second yield	1
4	Yield 2	2	Paused after yield 2	1 2
5	Resume function	N/A	At third yield	1 2
6	Yield 3	3	Paused after yield 3	1 2 3
7	Resume function	N/A	Function ends	1 2 3
8	Iteration ends	N/A	Done	1 2 3

💡 Function ends after yielding 3, iteration stops because no more yields.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
$number	undefined	1	2	3	undefined (iteration ended)

Key Moments - 3 Insights

Why does the function pause after yield instead of finishing?

Because yield returns a value and pauses the function, waiting for the caller to request the next value (see steps 2, 4, 6 in execution_table).

What happens when the caller asks for the next value?

The function resumes right after the last yield and continues until it hits the next yield or ends (see steps 3, 5, 7).

Why does the iteration stop after the last yield?

Because the function finishes execution with no more yields, so the iterator signals the end (step 8).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of $number after step 4?

A 1

B 2

C 3

D undefined

💡 Hint

Check the variable_tracker row for $number at After Step 4.

At which step does the function first pause and yield a value?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the execution_table Action and Function State columns.

If the function had an extra yield after step 6, how would the iteration end?

A It would continue with more steps until the new yield

B It would stop immediately after step 6

C It would end at step 8 as before

D It would cause an error

💡 Hint

Yield pauses and resumes function; more yields mean more steps (see concept_flow).

Concept Snapshot

PHP yield keyword pauses function execution and returns a value.
Each yield produces one value for the caller.
Function resumes after yield when caller requests next.
Iteration ends when function finishes with no more yields.
Useful for creating generators that produce values on demand.

Full Transcript

This visual trace shows how the PHP yield keyword works inside a function. When the function is called, it runs until it hits the first yield, which returns a value and pauses the function. The caller receives this value and can use it. When the caller asks for the next value, the function resumes right after the last yield and continues until it hits the next yield or ends. This repeats until the function finishes execution with no more yields, ending the iteration. Variables like $number take on the yielded values step by step. This behavior allows efficient generation of values one at a time without running the whole function at once.