The iterable pseudo-type in PHP allows a function to accept either an array or any object that can be looped over with foreach, such as those implementing Traversable or Generator. 1. It covers both arrays and traversable objects, enabling flexibility in input types. 2. It is useful when working with generators, reusable components, or when accepting data without needing array-specific features. 3. It should be used when the logic only needs to iterate over values, not access them via indexes or modify them. 4. Using iterable simplifies code by eliminating the need for separate handling of arrays and iterators. 5. Unlike Traversable, iterable includes arrays, making it more inclusive and readable. By using iterable, developers can write more adaptable and maintainable functions that work seamlessly across different data sources.

In PHP, the iterable pseudo-type isn't just a type hint — it's a flexible way to accept either an array or any object that can be looped over using foreach. This includes objects implementing Traversable, like those from Iterator or Generator. It makes writing functions more convenient when you don’t care specifically whether the data comes from an array or a custom iterator.

What does iterable actually cover?

When you type-hint a function parameter as iterable, you're saying:

• "I expect something I can loop through with foreach."

That means both of these will work:

• Arrays – because they’re naturally traversable.
• Objects implementing Traversable – which includes most built-in iterators and generators.

So if your function needs to process a sequence of values but doesn’t need to modify them or access them by index, using iterable gives you more flexibility than restricting yourself to just arrays.

When should you use iterable?

Here are some real-world cases where iterable shines:

• Accepting both arrays and iterators in one function: You might have a utility function that processes a list of items. Whether that list is an array or comes from a database cursor (like an iterator), your logic can stay the same.

• Working with generators: If your function receives data from a generator, you can't use an array type hint because generators return a Generator object, which is traversable but not an array.

• Writing reusable code: If you're building libraries or components where input types may vary, iterable helps keep things loosely coupled.

This is especially useful in frameworks or large applications where data sources may change over time, but the processing logic remains consistent.

How do you use iterable in practice?

Using iterable is straightforward. Here’s a basic example:

function processItems(iterable $items): void {
    foreach ($items as $item) {
        echo $item . PHP_EOL;
    }
}

Now you can pass either an array or a traversable object:

processItems([1, 2, 3]); // Works fine

$generator = function() {
    yield 1; yield 2; yield 3;
};

processItems($generator()); // Also works!

If you had used array instead of iterable, the second call would have thrown an error. So iterable gives you broader compatibility without having to write separate handling logic.

Just keep in mind:

• You can't use array access ($items[0]) on an iterable unless you know it's an array.
• You can't assume anything about internal structure — only that it can be looped.

Why not just use Traversable?

You might wonder why we don’t just type-hint with \Traversable. The reason is that arrays aren’t instances of Traversable, even though they can be iterated. So using iterable covers both arrays and Traversable objects under one simple keyword.

Also, PHP internally treats iterable as a union of array|\Traversable, so it's more inclusive and easier to read than manually combining types.

That’s basically it — iterable is a helpful shortcut that makes your code more flexible without forcing you into strict data format requirements.

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