Generator function execution model in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a generator function in PHP?

A generator function is a special function that can pause its execution and return values one at a time using the yield keyword, instead of returning all values at once.

beginner

How does the yield keyword affect the execution of a generator function?

The yield keyword pauses the function's execution and sends a value back to the caller. When the generator resumes, it continues from where it left off.

intermediate

What happens when a generator function is called in PHP?

Calling a generator function returns a Generator object immediately without running the function body. The function body runs only when iterating over the generator.

intermediate

Explain the difference between return and yield in a generator function.

return ends the generator and optionally sends a final value. yield pauses the generator and sends a value but allows resuming later to produce more values.

beginner

What is the benefit of using a generator function over returning an array?

Generators use less memory because they produce values one at a time on demand, instead of creating and storing the entire array in memory.

What does a generator function return when called in PHP?

A A Generator object

B An array of values

C The first yielded value

D Null

Calling a generator function returns a Generator object immediately without running the function body.

Which keyword is used to pause a generator function and send a value back?

A yield

B return

C break

D continue

The yield keyword pauses the generator and sends a value back to the caller.

What happens when the generator resumes after a yield?

A It restarts from the beginning

B It ends immediately

C It continues execution from the next statement after yield

D It throws an error

The generator resumes execution from the statement immediately after the yield.

Which of these is a key advantage of generators?

A Faster execution than normal functions always

B Lower memory usage by producing values on demand

C They can only return one value

D They automatically cache all results

Generators produce values one at a time, saving memory compared to building large arrays.

What does the return statement do inside a generator function?

A Skips the next yield

B Pauses the generator and sends a value

C Restarts the generator

D Ends the generator and optionally sends a final value

Return ends the generator function and can send a final value to the caller.

Describe how a generator function executes in PHP from the moment it is called until it finishes.

Think about the lifecycle of the generator and how yield controls execution.

Explain why using a generator function can be more memory efficient than returning an array.

Compare storing all data at once versus producing it step-by-step.

Practice

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

easy

A. It pauses the function and returns a value to the caller.

B. It immediately ends the function and returns a value.

C. It creates a new variable inside the function.

D. It calls another function asynchronously.

Solution

1. Step 1: Understand the role of yield in generators

   The yield keyword pauses the function execution and sends a value back to the caller without ending the function.

2. Step 2: Compare with other options

   Unlike return, yield does not end the function but allows it to resume later.

3. Final Answer:

   It pauses the function and returns a value to the caller. -> Option A

4. Quick Check:

   yield pauses and returns value [OK]

Hint: Remember: yield pauses, return ends function [OK]

Common Mistakes:

• Confusing yield with return
• Thinking yield creates variables
• Assuming yield calls other functions

2. Which of the following is the correct syntax to define a generator function in PHP?

easy

A. function gen() { return yield 1; }

B. function gen() { yield 1; }

C. function gen() => yield 1;

D. generator gen() { yield 1; }

Solution

1. Step 1: Identify correct generator syntax

   In PHP, a generator function is a normal function that uses yield inside its body.

2. Step 2: Check each option

   function gen() { yield 1; } uses a normal function with yield correctly. function gen() { return yield 1; } misuses return yield. function gen() => yield 1; uses invalid arrow syntax. generator gen() { yield 1; } uses an invalid keyword generator.

3. Final Answer:

   function gen() { yield 1; } -> Option B

4. Quick Check:

   Normal function + yield = generator [OK]

Hint: Generator = function with yield inside braces [OK]

Common Mistakes:

• Using return with yield incorrectly
• Trying arrow function syntax for generators
• Using non-existent keywords like 'generator'

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

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

medium

A. 123

B. Error: Cannot use foreach on generator

C. 1,2,3

D. 1 2 3

Solution

1. Step 1: Understand generator yields

   The generator yields values 1, then 2, then 3 in sequence.

2. Step 2: Analyze foreach output

   The foreach loops over each yielded value and echoes it followed by a space, so output is "1 2 3 "

3. Final Answer:

   1 2 3 -> Option D

4. Quick Check:

   foreach prints each yield with space [OK]

Hint: foreach prints each yield value in order [OK]

Common Mistakes:

• Ignoring spaces in echo output
• Thinking foreach can't iterate generators
• Confusing output format with commas

4. Identify the error in this PHP generator function:

function gen() {
  yield 1;
  return 2;
  yield 3;
}
foreach (gen() as $v) {
  echo $v;
}

medium

A. Syntax error: return not allowed in generator

B. No error, outputs 123

C. The return ends generator early, so yield 3 is never reached

D. foreach cannot iterate generator with return

Solution

1. Step 1: Understand return in generators

   Return ends the generator function immediately, so code after return is not executed.

2. Step 2: Analyze code flow

   After yielding 1, the return 2 ends the generator, so yield 3 is never reached or yielded.

3. Final Answer:

   The return ends generator early, so yield 3 is never reached -> Option C

4. Quick Check:

   Return stops generator, later yields skipped [OK]

Hint: Return ends generator; yields after return ignored [OK]

Common Mistakes:

• Thinking return causes syntax error
• Expecting all yields to run despite return
• Assuming foreach can't handle return in generator

5. Given this generator function, what will be the output?

function gen() {
  for ($i = 0; $i < 3; $i++) {
    yield $i => $i * 2;
  }
}
foreach (gen() as $key => $value) {
  echo "$key:$value ";
}

hard

A. 0:0 1:2 2:4

B. 0:0 1:1 2:2

C. 0 2 4

D. Error: Cannot yield key => value in PHP

Solution

1. Step 1: Understand yield with key => value

   The generator yields keys 0,1,2 with values 0,2,4 respectively (value = key * 2).

2. Step 2: Analyze foreach output

   foreach prints each key and value separated by colon and space, so output is "0:0 1:2 2:4 "

3. Final Answer:

   0:0 1:2 2:4 -> Option A

4. Quick Check:

   Yield key=>value prints key:value pairs [OK]

Hint: Yield key=>value pairs print as key:value [OK]

Common Mistakes:

• Confusing value with key
• Thinking yield key=>value is invalid
• Ignoring spaces in output