Intersection & DNF Types in Practice

TL;DR

Explanation

Intersection types (PHP 8.1): function process(Countable&Iterator $c) — $c must implement BOTH interfaces. Only works with class/interface types, not scalars. DNF (Disjunctive Normal Form) types (PHP 8.2): (Countable&Iterator)|null — unions of intersections. The DNF form requires intersections in parentheses inside unions. Use cases: utility functions that require an object to implement multiple interfaces, nullable intersection types. PHPStan and Psalm both support intersection types for static analysis.

Common Misconception

Why It Matters

Common Mistakes

• Using intersection with scalar types — fatal error.
• Not using DNF parentheses for (A&B)|null — required syntax in PHP 8.2.
• Confusing union (A|B = either) with intersection (A&B = both).

Code Examples

// Can't use intersection with scalars:
function foo(int&string $x) {} // Fatal Error

interface Serializable {}
interface Loggable {}

// Must implement BOTH:
function process(Serializable&Loggable $obj): void {}

// PHP 8.2 DNF — nullable intersection:
function maybe((Serializable&Loggable)|null $obj): void {}

References

• ↗ https://www.php.net/manual/en/language.types.type-system.php

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