Basic class definitions begin with the keyword class, followed by a class name, followed by a pair of curly braces which enclose the definitions of the properties and methods belonging to the class. The class name can be any valid label, provided it is not a PHP reserved word. A valid class name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: ^[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*$. A class may contain its own constants, variables (called "properties"), and functions (called "methods"). var; } } ?> ]]> The pseudo-variable $this is available when a method is called from within an object context. $this is the value of the calling object. Calling a non-static method statically throws an Error. Prior to PHP 8.0.0, this would generate a deprecation notice, and $this would be undefined. foo(); A::foo(); $b = new B(); $b->bar(); B::bar(); ?> ]]> &example.outputs.7; &example.outputs.8; To create an instance of a class, the new keyword must be used. An object will always be created unless the object has a constructor defined that throws an exception on error. Classes should be defined before instantiation (and in some cases this is a requirement). If a string containing the name of a class is used with new, a new instance of that class will be created. If the class is in a namespace, its fully qualified name must be used when doing this. If there are no arguments to be passed to the class's constructor, parentheses after the class name may be omitted. ]]> In the class context, it is possible to create a new object by new self and new parent. When assigning an already created instance of a class to a new variable, the new variable will access the same instance as the object that was assigned. This behaviour is the same when passing instances to a function. A copy of an already created object can be made by cloning it. var = '$assigned will have this value'; $instance = null; // $instance and $reference become null var_dump($instance); var_dump($reference); var_dump($assigned); ?> ]]> &example.outputs; string(30) "$assigned will have this value" } ]]> It's possible to create instances of an object in a couple of ways: ]]> &example.outputs; It is possible to access a member of a newly created object in a single expression: format('Y'); ?> ]]> &example.outputs.similar; Prior to PHP 7.1, the arguments are not evaluated if there is no constructor function defined. Class properties and methods live in separate "namespaces", so it is possible to have a property and a method with the same name. Referring to both a property and a method has the same notation, and whether a property will be accessed or a method will be called, solely depends on the context, i.e. whether the usage is a variable access or a function call. bar, PHP_EOL, $obj->bar(), PHP_EOL; ]]> &example.outputs; That means that calling an anonymous function which has been assigned to a property is not directly possible. Instead the property has to be assigned to a variable first, for instance. It is possible to call such a property directly by enclosing it in parentheses. bar = function() { return 42; }; } } $obj = new Foo(); echo ($obj->bar)(), PHP_EOL; ]]> &example.outputs; A class can inherit the constants, methods, and properties of another class by using the keyword extends in the class declaration. It is not possible to extend multiple classes; a class can only inherit from one base class. The inherited constants, methods, and properties can be overridden by redeclaring them with the same name defined in the parent class. However, if the parent class has defined a method as final, that method may not be overridden. It is possible to access the overridden methods or static properties by referencing them with parent::. displayVar(); ?> ]]> &example.outputs; When overriding a method, its signature must be compatible with the parent method. Otherwise, a fatal error is emitted, or, prior to PHP 8.0.0, an E_WARNING level error is generated. A signature is compatible if it respects the variance rules, makes a mandatory parameter optional, and if any new parameters are optional. This is known as the Liskov Substitution Principle, or LSP for short. The constructor, and private methods are exempt from these signature compatibility rules, and thus won't emit a fatal error in case of a signature mismatch. foo(); $extended2 = new Extend2(); $extended2->foo(1); ]]> &example.outputs; The following examples demonstrate that a child method which removes a parameter, or makes an optional parameter mandatory, is not compatible with the parent method. &example.outputs.8.similar; &example.outputs.8.similar; Renaming a method's parameter in a child class is not a signature incompatibility. However, this is discouraged as it will result in a runtime Error if named arguments are used. test(foo: "foo", bar: "bar"); // ERROR! ]]> &example.outputs.similar; The class keyword is also used for class name resolution. To obtain the fully qualified name of a class ClassName use ClassName::class. This is particularly useful with namespaced classes. ]]> &example.outputs; The class name resolution using ::class is a compile time transformation. That means at the time the class name string is created no autoloading has happened yet. As a consequence, class names are expanded even if the class does not exist. No error is issued in that case. ]]> &example.outputs; As of PHP 8.0.0, the ::class constant may also be used on objects. This resolution happens at runtime, not compile time. Its effect is the same as calling get_class on the object. ]]> &example.outputs; As of PHP 8.0.0, properties and methods may also be accessed with the "nullsafe" operator instead: ?->. The nullsafe operator works the same as property or method access as above, except that if the object being dereferenced is &null; then &null; will be returned rather than an exception thrown. If the dereference is part of a chain, the rest of the chain is skipped. The effect is similar to wrapping each access in an is_null check first, but more compact. getUser(5)?->name; // Is equivalent to the following code block: if (is_null($repository)) { $result = null; } else { $user = $repository->getUser(5); if (is_null($user)) { $result = null; } else { $result = $user->name; } } ?> ]]> The nullsafe operator is best used when null is considered a valid and expected possible value for a property or method return. For indicating an error, a thrown exception is preferable.