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Metaprogramming

Metaprogramming is the practice of writing code that treats other code as data: reading, generating, transforming, or executing programs at runtime or compile time. A metaprogram operates at a higher level of abstraction than an ordinary program because it manipulates the representation of programs rather than, or in addition to, running domain logic.

Common metaprogramming techniques include:

  • Reflection: inspecting types, methods, properties, or annotations at runtime, with the ReflectionClass
  • Code generation: emitting source files, proxy classes, or configuration from schemas, attributes, or templates
  • Macros: transforming code at a syntactic level before or during parsing
  • Eval: constructing and executing code strings at runtime
  • Dynamic dispatch: routing method calls through __call or __callStatic based on runtime state
  • Homoiconicity: using a language’s native data structures to represent and manipulate programs

PHP supports several forms of metaprogramming:

  • The Reflection API, with the classes ReflectionClass, ReflectionMethod, ReflectionProperty, ReflectionAttribute, provides read-only introspection of the live object model.
  • Magic methods, with the methods __get, __set, __call, __callStatic, __invoke, __clone, __toString, intercept property and method access at runtime.
  • eval() and create_function() execute dynamically constructed code strings.
  • nikic/php-parser enables full AST-level read and write access to PHP source from PHP itself.
  • PHP 8.0+ attributes allow structured metadata on declarations, consumed at runtime by frameworks and code generators.
  • Userland code generators, such as Doctrine proxies, Rector, Symfony DI compilation, etc. produce PHP files before or at application boot.

PHP does not support compile-time generics, hygienic macros, metaclasses, or a Meta-object Protocol: the object model is implemented in C and is not programmable from userland.

<?php

    // 1. Reflection: introspect a class at runtime
    $rc = new ReflectionClass(ArrayObject::class);
    echo $rc->getMethod('offsetGet')->getNumberOfParameters(); // 1

    // 2. Dynamic dispatch via magic method
    class DynamicProxy {
        public function __call(string $name, array $args): mixed {
            return strtoupper($name) . '(' . implode(', ', $args) . ')';
        }
    }
    echo (new DynamicProxy())->hello('world'); // HELLO(world)

    // 3. Attribute-driven metadata (PHP 8.0+)
    #[\Attribute(\Attribute::TARGET_METHOD)]
    final class Route {
        public function __construct(public readonly string $path) {}
    }

    class UserController {
        #[Route('/users')]
        public function index(): void {}
    }

    $attr = (new ReflectionMethod(UserController::class, 'index'))
        ->getAttributes(Route::class)[0]
        ->newInstance();
    echo $attr->path; // '/users'

?>

Documentation

See Also