IteratorAggregate interface in PHP - Step-by-Step Execution

Concept Flow - IteratorAggregate interface

Create class implementing IteratorAggregate

↓

Define getIterator() method

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Inside getIterator(), return an Iterator object

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Use foreach on class instance

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foreach calls getIterator()

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Iterate over returned Iterator

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Access each element in loop

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End

This flow shows how a class implements IteratorAggregate by defining getIterator() which returns an iterator used by foreach to loop over elements.

Execution Sample

PHP

<?php
class MyCollection implements IteratorAggregate {
  private array $items = ["apple", "banana", "cherry"];
  public function getIterator(): Traversable {
    return new ArrayIterator($this->items);
  }
}
foreach (new MyCollection() as $item) {
  echo $item . "\n";
}

This code defines a class that returns an iterator over an array, then loops over it printing each item.

Execution Table

Step	Action	Code Line	Result/Output
1	Create MyCollection instance	$collection = new MyCollection();	Object created with items ["apple", "banana", "cherry"]
2	foreach starts looping over $collection	foreach (new MyCollection() as $item)	Calls getIterator() method
3	Call getIterator()	public function getIterator()	Returns ArrayIterator over items
4	First iteration	echo $item . "\n";	Outputs: apple
5	Second iteration	echo $item . "\n";	Outputs: banana
6	Third iteration	echo $item . "\n";	Outputs: cherry
7	No more items	foreach ends	Loop ends, iteration stops

💡 Iteration stops when ArrayIterator has no more elements.

Variable Tracker

Variable	Start	After Step 4	After Step 5	After Step 6	Final
$items	["apple", "banana", "cherry"]	["apple", "banana", "cherry"]	["apple", "banana", "cherry"]	["apple", "banana", "cherry"]	["apple", "banana", "cherry"]
$item	undefined	apple	banana	cherry	undefined (loop ended)

Key Moments - 3 Insights

Why does foreach call getIterator() on the object?

Because the class implements IteratorAggregate, foreach looks for getIterator() to get an iterator to loop over, as shown in execution_table step 2 and 3.

What type of object must getIterator() return?

It must return an object implementing Traversable, like ArrayIterator, so foreach can iterate over it (see step 3).

Why does $item change each iteration?

$item gets assigned the current element from the iterator each loop cycle, shown in steps 4, 5, and 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output at step 5?

A apple

B banana

C cherry

D No output

💡 Hint

Check the 'Result/Output' column for step 5 in execution_table.

At which step does foreach call getIterator()?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column describing foreach starting and calling getIterator().

If getIterator() returned a non-iterator object, what would happen?

A Loop would run zero times silently

B foreach would loop normally

C PHP would throw an error

D Output would be empty strings

💡 Hint

Recall that getIterator() must return Traversable; otherwise, foreach cannot iterate (see key_moments).

Concept Snapshot

IteratorAggregate interface:
- Implement IteratorAggregate in your class
- Define getIterator() method
- getIterator() returns an iterator (implements Traversable)
- foreach uses getIterator() to loop
- Enables custom iteration over objects

Full Transcript

This example shows how a PHP class uses the IteratorAggregate interface to allow foreach loops over its data. The class defines getIterator() which returns an ArrayIterator over an internal array. When foreach runs, it calls getIterator() to get the iterator, then loops over each element, assigning it to the loop variable and printing it. The execution table traces each step: creating the object, calling getIterator(), and each iteration output. Variables like $item change each loop. Key points include that foreach calls getIterator() automatically on objects implementing IteratorAggregate, and getIterator() must return an iterator object. If it doesn't, PHP throws an error. This pattern helps make objects iterable in a simple, flexible way.