Yield keyword behavior in PHP

Introduction

The yield keyword helps create simple functions that produce values one at a time, saving memory and making code easier to read.

When you want to process large lists without loading everything into memory at once.

When you need to generate a sequence of values step-by-step, like numbers or data from a file.

When you want to pause a function and resume it later to produce the next value.

When you want to write cleaner code instead of building and returning big arrays.

When you want to improve performance by producing values on demand.

Syntax

PHP

<?php
function generatorFunction() {
    yield 1;
}

yield pauses the function and sends a value back to the caller.

Each time the generator is called again, it resumes right after the last yield.

Examples

This function yields numbers 1, 2, and 3 one by one.

PHP

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

This generator yields letters instead of numbers.

PHP

<?php
function letters() {
    yield 'a';
    yield 'b';
    yield 'c';
}

This generator yields squares of numbers from 1 up to $max.

PHP

<?php
function squares($max) {
    for ($i = 1; $i <= $max; $i++) {
        yield $i * $i;
    }
}

Sample Program

This program uses a generator to yield two words, printing each on its own line.

PHP

<?php
function simpleGenerator() {
    yield 'Hello';
    yield 'World';
}

foreach (simpleGenerator() as $word) {
    echo $word . "\n";
}

Important Notes

Generators created with yield are memory efficient because they don't build the whole list at once.

You can use foreach to loop over values yielded by the generator.

Generators can also receive values back using yield with keys or by sending values, but that is more advanced.

Summary

Yield lets functions produce values one at a time.

It helps save memory and write cleaner code for sequences.

Use foreach to get each yielded value step-by-step.

Practice

1. What does the yield keyword do in a PHP function?

easy

A. It returns a value and pauses the function, resuming later to produce more values.

B. It immediately stops the function and returns a single value.

C. It creates a new variable inside the function.

D. It converts the function into a class method automatically.

Solution

1. Step 1: Understand yield behavior

   The yield keyword pauses the function and returns a value without ending it.

2. Step 2: Compare with return

   Unlike return, yield allows the function to resume and produce more values later.

3. Final Answer:

   It returns a value and pauses the function, resuming later to produce more values. -> Option A

4. Quick Check:

   Yield pauses and resumes function = A [OK]

Hint: Yield pauses function to produce values one by one [OK]

Common Mistakes:

• Confusing yield with return which ends function
• Thinking yield creates variables
• Assuming yield converts function to class method

2. Which of the following is the correct syntax to yield values from a function in PHP?

easy

A. yield($value);

B. return yield $value;

C. yield => $value;

D. yield $value;

Solution

1. Step 1: Recall yield syntax

   The correct syntax to yield a value is simply yield $value; inside the function.

2. Step 2: Check other options

   Options B, C, and D use invalid or incorrect syntax for yield in PHP.

3. Final Answer:

   yield $value; -> Option D

4. Quick Check:

   Simple yield syntax is 'yield $value;' [OK]

Hint: Yield uses simple 'yield $value;' syntax [OK]

Common Mistakes:

• Adding return before yield
• Using arrow (=>) with yield
• Calling yield like a function

3. What will be the output of this PHP code?

function gen() {
  yield 1;
  yield 2;
  yield 3;
}
foreach (gen() as $val) {
  echo $val . ' ';
}

medium

A. 1 2 3

B. Array

C. 123

D. Error: yield not allowed here

Solution

1. Step 1: Understand generator yields

   The function gen() yields values 1, 2, and 3 one by one.

2. Step 2: foreach prints each yielded value

   The foreach loop prints each value followed by a space, so output is '1 2 3 '.

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   Yielded values printed with spaces = B [OK]

Hint: Yield outputs values one by one in foreach loop [OK]

Common Mistakes:

• Expecting concatenated string without spaces
• Thinking yield returns an array
• Assuming syntax error with yield

4. Identify the error in this PHP code using yield:

function numbers() {
  yield 1
  yield 2;
}
foreach(numbers() as $n) {
  echo $n;
}

medium

A. foreach loop syntax is wrong

B. Missing semicolon after first yield statement

C. Cannot use yield inside a function

D. Yield must be followed by parentheses

Solution

1. Step 1: Check yield statements

   The first yield statement is missing a semicolon at the end, causing a syntax error.

2. Step 2: Validate other parts

   Using yield inside a function and foreach syntax are correct; yield does not require parentheses.

3. Final Answer:

   Missing semicolon after first yield statement -> Option B

4. Quick Check:

   Each yield needs semicolon [OK]

Hint: Check semicolons after yield statements [OK]

Common Mistakes:

• Forgetting semicolon after yield
• Thinking yield needs parentheses
• Assuming foreach syntax error

5. Given this PHP function using yield:

function filterPositive(array $numbers) {
  foreach ($numbers as $num) {
    if ($num > 0) {
      yield $num;
    }
  }
}

$values = [-2, 0, 3, 5, -1];
foreach (filterPositive($values) as $val) {
  echo $val . ',';
}

What is the output?

hard

A. 0,3,5,

B. -2,0,3,5,-1,

C. 3,5,

D. Error: yield cannot be used with if

Solution

1. Step 1: Analyze filtering condition

   The function yields only numbers greater than 0, so it skips -2, 0, and -1.

2. Step 2: Identify yielded values

   Only 3 and 5 satisfy the condition and are yielded, printed with commas.

3. Final Answer:

   3,5, -> Option C

4. Quick Check:

   Yield filters positive numbers only = D [OK]

Hint: Yield can filter values with if condition [OK]

Common Mistakes:

• Including zero or negative numbers
• Thinking yield can't be inside if
• Expecting all values printed