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 which is a 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_\x7f-\xff][a-zA-Z0-9_\x7f-\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 a reference to the calling object (usually the object to which the method belongs, but possibly another object, if the method is called statically from the context of a secondary object). foo(); // Note: the next line will issue a warning if E_STRICT is enabled. A::foo(); $b = new B(); $b->bar(); // Note: the next line will issue a warning if E_STRICT is enabled. B::bar(); ?> ]]> &example.outputs; To create an instance of a class, the new keyword must be used. An object will always be created unless the object has a constructor defined that throws an exception on error. Classes should be defined before instantiation (and in some cases this is a requirement). If a string containing the name of a class is used with new, a new instance of that class will be created. If the class is in a namespace, its fully qualified name must be used when doing this. ]]> In the class context, it is possible to create a new object by new self and new parent. When assigning an already created instance of a class to a new variable, the new variable will access the same instance as the object that was assigned. This behaviour is the same when passing instances to a function. A copy of an already created object can be made by cloning it. var = '$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" } ]]> PHP 5.3.0 introduced a couple of new ways to create instances of an object: ]]> &example.outputs; A class can inherit the 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 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::. When overriding methods, the parameter signature should remain the same or PHP will generate an E_STRICT level error. This does not apply to the constructor, which allows overriding with different parameters. displayVar(); ?> ]]> &example.outputs;