Yield keyword behavior in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the yield keyword do in PHP?

The yield keyword allows a function to return a value to the caller without losing its state, enabling the function to be resumed later to produce a sequence of values one at a time.

beginner

How is yield different from return in PHP?

return ends the function and sends a single value back, while yield pauses the function, sends a value back, and allows the function to continue from where it left off when called again.

intermediate

What type of object does a function using yield return in PHP?

It returns a Generator object, which can be iterated over to get each yielded value one by one.

intermediate

Can yield send keys along with values? How?

Yes, you can use yield key => value; to send both a key and a value, which is useful when iterating over the generator with keys.

intermediate

Why use yield instead of building an array in PHP?

Using yield saves memory because it generates values one at a time instead of creating a full array in memory, which is helpful for large data sets or infinite sequences.

What does the yield keyword return when used inside a PHP function?

A A Generator object

B An array

C A string

D A boolean

yield returns a Generator object that can be iterated to get each yielded value.

Which statement about yield is true?

A <code>yield</code> pauses the function and returns a value

B <code>yield</code> immediately ends the function

C <code>yield</code> returns all values at once

D <code>yield</code> is used to declare variables

yield pauses the function and returns a value, allowing the function to resume later.

How do you send a key and value using yield?

A yield value => key;

B yield (key, value);

C yield key => value;

D yield key, value;

The syntax yield key => value; sends both a key and a value.

Why might you prefer yield over returning an array?

A It converts values to strings

B It runs faster because it returns all values immediately

C It automatically sorts the values

D It uses less memory by generating values one at a time

yield generates values one at a time, saving memory compared to building a full array.

What happens if you call a function with yield multiple times?

A It returns the last yielded value only

B It produces a sequence of values, one per call to the generator

C It throws an error

D It returns an array of all yielded values

Each call to the generator's next() method resumes the function and produces the next yielded value.

Explain how the yield keyword works in PHP and how it differs from return.

Think about how <code>yield</code> lets a function produce multiple values over time.

Describe a situation where using yield is better than returning an array.

Consider when you have many values but don't want to store them all at once.

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