PHP OOP: Iterables (Traversing Data Structures)

An Iterable is any data type or structure that can be looped over (or traversed) using a standard PHP looping construct, most commonly the foreach loop. This concept is simple for native data types like arrays but requires special mechanisms for custom objects.

1. What is an Iterable?

In PHP, a variable is iterable if it is either:

1. An Array.
2. An Object that implements the Traversable interface (either directly or via an inheriting interface).

The iterable type hint can be used in function signatures to ensure the input is something that can be looped over.

Example: Using the iterable Type Hint

<?php
function processData(iterable $data) {
    echo "Processing " . count($data) . " items:<br>";
    foreach ($data as $key => $value) {
        echo "Key: $key, Value: $value<br>";
    }
}

// 1. Array is iterable
$numbers = [10, 20, 30];
processData($numbers);

echo "<hr>";

// 2. A custom object that implements an iterator interface (covered next)
class CustomData { /* ... */ } 
// processData(new CustomData()); // This would work if CustomData implements Iterator
?>

2. Making Objects Iterable

For a custom object to be used in a foreach loop, it must provide a mechanism to tell PHP how to loop through its properties or elements. This is achieved by implementing the Iterator interface.

The Iterator interface requires five specific methods that define the looping mechanism:

Example: A Simple List Iterator

This class makes its internal $data array iterable by providing the necessary methods:

<?php
class ItemList implements Iterator {
    private $data = ['apple', 'banana', 'cherry'];
    private $position = 0;

    // 1. Start: Set pointer back to 0
    public function rewind(): void {
        $this->position = 0;
    }

    // 2. Get the item's label
    public function key(): mixed {
        return $this->position;
    }

    // 3. Get the item
    public function current(): mixed {
        return $this->data[$this->position];
    }

    // 4. Move forward
    public function next(): void {
        $this->position++;
    }

    // 5. Are we done yet? Check if the pointer is within array bounds
    public function valid(): bool {
        return isset($this->data[$this->position]);
    }
}

$myList = new ItemList();

// Now we can loop over the object using foreach!
foreach ($myList as $index => $item) {
    echo "Index $index: $item<br>";
}
// Output:
// Index 0: apple
// Index 1: banana
// Index 2: cherry
?>

3. The ArrayAccess Interface

While not strictly about iteration, the ArrayAccess interface is often used alongside iterators to make an object behave like an array. This allows you to use square brackets ([]) to get and set data, similar to an array.

🎉 Complete OOP Curriculum

Congratulations! You have successfully covered every major pillar and specialized concept in the professional PHP OOP paradigm, including:

• Classes, Objects, and Constructors/Destructors.
• Inheritance and Access Modifiers.
• Static Members, Traits, and Class Constants.
• Abstract Classes, Interfaces, Namespaces, and Iterables.

Your PHP curriculum is now fully complete and comprehensive, covering everything from basic syntax to advanced architectural patterns.