Object-Oriented PHP

Since PHP 8.3 a class constant can declare a type, and a subclass override must keep a compatible type. self::NAME reads the constant of the class the code is in; static::NAME reads the called class's value, so a subclass override takes effect.

Yes. An inherited static property that the subclass does not redeclare is the same storage as the parent's, so writes through Child::$x change Parent::$x too. Redeclare it in the subclass, or key an array by static::class, to get per-class values.

Abstract trait methods are requirements the host must implement compatibly. Static state exists once per using class, not per trait. A host may redeclare a trait property or constant only identically, and trait constants (8.2) and static members are used through the using class.

Two traits defining the same concrete method make PHP fail until insteadof picks one. The as operator adds an alias so the excluded body stays callable, and can also change visibility or, since 8.3, make the method final.

__destruct runs as soon as the last handle to an object disappears, or at shutdown if the object is still alive. Fatal errors skip it, cycles delay it and shutdown order is undefined, so commit work explicitly.

The stored ->value is the contract. Renaming a case breaks code and anything that stored the name; changing a value makes from() throw ValueError on old rows; adding a case makes every default-less match throw UnhandledMatchError until handled.

Since PHP 8.5, clone($obj, ['prop' => $value]) shallow-copies the object, runs __clone(), then assigns the listed properties on the copy, even readonly ones, respecting visibility and set hooks. Withers become one line.

Since PHP 8.4 an interface can require a public readable, writable or read-write property with { get; }, { set; } or both. Implementers may use a plain public property, a hooked or virtual property, or, for get-only, a readonly one.

Magic __get hides every property name and type from tools, so typos, renames and wrong types surface only at runtime. For known keys use typed readonly properties or 8.4 property hooks; if magic stays, document it with @property-read tags.

Since PHP 8.4, get and set hooks attach logic to a property. A property whose hooks never touch its own storage is virtual and takes no memory. Hooked properties cannot be readonly, so readonly classes cannot declare hooks.

Write create() as return new static(...) with a : static return type, so User::create() builds a User. Guard the constructor signature (final, abstract or interface-declared), because a subclass constructor with different parameters makes new static throw.

A trait fits small, cohesive behaviour that several unrelated classes need verbatim, such as timestamp and soft-delete fields. Keep it honest by pairing it with an interface, declaring its needs as abstract methods, and leaving real dependencies to injected services.

An anonymous class is declared and instantiated in one new class expression; it can take constructor arguments, extend, implement and use traits. It suits one-off implementations such as test doubles or small adapters, not reusable types.

Stringable, added in PHP 8.0, is the interface of objects with __toString(). A class that declares __toString() implements it automatically. Typing string|Stringable accepts strings and such objects even under strict_types, where a plain string type rejects objects.

__debugInfo returns the array var_dump and print_r show for an object, for example to hide a secret. __set_state is a static factory that var_export's output calls when that code is evaluated, rebuilding the object from an array of properties.

Magic methods have fixed parameter counts, no by-reference parameters, and only __callStatic and __set_state may be static. Since PHP 8.0, declared types must also match the documented signatures. A non-public magic method just triggers a warning.

Since PHP 8.1 most non-final internal methods carry tentative return types; an override without a compatible one raises E_DEPRECATED. Declaring the type fixes it; #[\ReturnTypeWillChange] only suppresses the notice until checking becomes strict.