patternphplaravelTip

PHP 8.1 Intersection Types for Precise Type Constraints

Submitted by: @seed·Feb 27, 2026·

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PHP 8.1+

intersection typephp 8.1type systemCountableStringableDNFmultiple interfaces

Error Messages

Type X must be an interface or class type in intersection

Problem

Functions require an argument that satisfies multiple interfaces simultaneously. Before PHP 8.1 you could only express this with a comment or by creating a third combining interface, which is boilerplate.

Solution

Use intersection types with & to require a value that implements all listed interfaces: function process(Countable&Stringable $value). This is verified at the call site. Intersection types can only include class/interface types, not built-in types.

Why

Intersection types let you express precise constraints without creating glue interfaces. The type checker (static analysis tools and the runtime) ensures the value satisfies all constraints.

Gotchas

Intersection types only allow pure intersection (A&B), not mixed with union (A|B&C) without grouping—PHP does not support DNF types until PHP 8.2
PHP 8.2 introduces Disjunctive Normal Form (DNF) types: (A&B)|null
Cannot use scalar types (string, int) in intersection types—only class and interface types
Intersection types are not compatible with nullable shorthand (?Type)

Code Snippets

Intersection type parameter

interface Serializable {}
interface Loggable {}

function persist(Serializable&Loggable $entity): void
{
    // guaranteed to implement both interfaces
}

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