Why Intersection types in practice in PHP? - Purpose & Use Cases

The Big Idea

What if you could tell PHP exactly what combination of features an object must have, and it would check for you automatically?

The Scenario

Imagine you have a function that needs an object which is both a Logger and a FileHandler. Without intersection types, you must manually check and combine these features, writing extra code to ensure the object meets both needs.

The Problem

This manual approach is slow and error-prone because you have to write repetitive checks and casts. It's easy to forget one requirement, causing bugs that are hard to find. The code becomes messy and hard to maintain.

The Solution

Intersection types let you declare that a parameter must satisfy multiple types at once. This means PHP can enforce the object has all needed features, making your code cleaner, safer, and easier to understand.

Before vs After

✗ Before

$obj = getObject();
if (!($obj instanceof Logger) || !($obj instanceof FileHandler)) {
    throw new Exception('Invalid object');
}
$obj->log('message');

✓ After

function process(Logger&FileHandler $obj) {
    $obj->log('message');
}

What It Enables

It enables writing functions that clearly require objects combining multiple roles, improving code safety and expressiveness.

Real Life Example

For example, a system that logs errors to a file needs an object that is both a Logger and a FileHandler. Intersection types let you require exactly that, avoiding runtime errors.

Key Takeaways

Manual checks for multiple roles are slow and error-prone.

Intersection types let PHP enforce multiple type requirements at once.

This leads to cleaner, safer, and more understandable code.