Why PHP Cannot Use Generics: Exploring Monomorphization, Reification, and Type Erasure

PHP does not support generics as a first‑class language feature, and this article explores the reasons behind that limitation.

Three common ways to implement generics in other languages are examined:

Monomorphization (single‑type generics) – the compiler creates a separate concrete class for each type parameter. The article shows a manual version using separate collection classes and then demonstrates how PHP could simulate it with code like $users = new Collection<User>(); // Collection_User and

$slugs = new Collection<string>(); // Collection_string

. While functional, this approach incurs significant memory overhead because multiple copies of the same class are generated.

Reification (concrete generics) – the generic type is retained at runtime and checked dynamically, similar to C# or Kotlin. Implementing this in PHP would require extensive core refactoring and would add runtime type‑checking costs.

Type erasure (ignore generics at runtime) – the generic information is discarded, as done in Java and Python. The article highlights problems this causes in PHP, such as loss of type safety when adding values to a Collection<string> and the inability to warn about mismatched types because PHP lacks a static analysis step.

Examples illustrate how PHP’s dynamic type system can lead to subtle bugs, e.g., a function function add(int $a, int $b): int { return $a + $b; } called with string arguments works, but adding an integer to a Collection<string> would not be automatically converted.

The article argues that most benefits of generics come from static analysis rather than runtime checks, and PHP’s lack of a compile‑time type checker makes generic support less valuable.

Ultimately, the primary reason PHP cannot feasibly add generics is that runtime verification of generic types would be too complex and resource‑intensive for the language’s execution model.

English original: https://stitcher.io/blog/generics-in-php-3