In PHP, an iterable is a data structure that can be looped over or iterated. It refers to any object or data type that can be used in a `foreach` loop to traverse its elements one by one. Iterables can include arrays, objects implementing the `Traversable` interface, and any other custom data structure that allows iteration.

The `foreach` loop is the primary construct used to iterate over iterables. It provides a simple way to loop through the elements of an iterable without worrying about the internal structure of the data.

Here's an example of using a `foreach` loop with an array:

php
$fruits = ['apple', 'orange', 'banana'];

foreach ($fruits as $fruit) {
    echo $fruit . ' ';
}
// Output: apple orange banana

In the above example, `$fruits` is an array, which is an iterable, and we use the `foreach` loop to iterate through its elements.

PHP 7.1 introduced the `iterable` pseudo-type. It allows you to declare function or method parameters and return types as "iterable," which means that the parameter can accept any iterable data structure. Similarly, a function or method can return an iterable without specifying a specific data type.

Here's an example of using the `iterable` pseudo-type:

php
function processIterable(iterable $data) {
    foreach ($data as $item) {
        echo $item . ' ';
    }
}

$fruits = ['apple', 'orange', 'banana'];
processIterable($fruits);
// Output: apple orange banana

In the above example, the `processIterable` function takes an iterable parameter named `$data` and uses a `foreach` loop to process its elements.

Using the `iterable` pseudo-type in function or method declarations allows you to create more flexible and reusable code that can work with various types of iterables, such as arrays or objects implementing the `Traversable` interface.

Remember that when working with custom objects, you need to ensure that the object implements the `Traversable` interface or provides a custom iterator to enable iteration over its elements in a `foreach` loop.

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