Open Class

Open classes allow external code to add or replace methods on an existing class after it has been defined, including built-in or third-party classes. This is sometimes called controlled monkey-patching when the language provides guard-rails to avoid unintended conflicts.

Languages such as Ruby expose this feature natively: any class can be reopened with class X ... end and new methods are merged in. The class remains open throughout the program’s lifetime, so any part of the codebase can contribute methods.

PHP does not support open classes. Once a class is defined its set of methods is fixed. The closest approximations are:

  • Traits, which must be composed at the point of class definition (not after).

  • __call / __callStatic magic methods, which dispatch unknown method calls dynamically but without type-level visibility.

  • Wrapper or decorator classes that delegate to the original.

None of these allow retrofitting a method onto a class that has already been fully declared.

<?php

// PHP does not support reopening a class.
// The following is illustrative of what the feature would look like,
// but it is a syntax error in PHP.

class MyString {
    public function upper(): string { return strtoupper($this->value); }
}

// Hypothetical open-class syntax (NOT valid PHP):
// open class MyString {
//     public function lower(): string { return strtolower($this->value); }
// }

?>

See also Ruby open classes.

Related : Trait, Use In Traits, __call() Method, __callStatic() Method, Mixin