As of PHP 5.4.0, PHP implements a method of code reuse called Traits. Traits is a mechanism for code reuse in single inheritance languages such as PHP. A Trait is intended to reduce some limitations of single inheritance by enabling a developer to reuse sets of methods freely in several independent classes living in different class hierarchies. The semantics of the combination of Traits and classes is defined in a way which reduces complexity, and avoids the typical problems associated with multiple inheritance and Mixins. A Trait is similar to a class, but only intended to group functionality in a fine-grained and consistent way. It is not possible to instantiate a Trait on its own. It is an addition to traditional inheritance and enables horizontal composition of behavior; that is, the application of class members without requiring inheritance. ]]> An inherited member from a base class is overridden by a member inserted by a Trait. The precedence order is that members from the current class override Trait methods, which in return override inherited methods. An inherited method from a base class is overridden by the method inserted into MyHelloWorld from the SayWorld Trait. The behavior is the same for methods defined in the MyHelloWorld class. The precedence order is that methods from the current class override Trait methods, which in turn override methods from the base class. sayHello(); ?> ]]> &example.outputs; sayHello(); ?> ]]> &example.outputs; Multiple Traits can be inserted into a class by listing them in the use statement, separated by commas. sayHello(); $o->sayWorld(); $o->sayExclamationMark();> ?> ]]> &example.outputs; If two Traits insert a method with the same name, a fatal error is produced, if the conflict is not explicitly resolved. To resolve naming conflicts between Traits used in the same class, the insteadof operator needs to be used to chose exactly one of the conflicting methods. Since this only allows one to exclude methods, the as operator can be used to allow the inclusion of one of the conflicting methods under another name. In this example, Talker uses the traits A and B. Since A and B have conflicting methods, it defines to use the variant of smallTalk from trait B, and the variant of bigTalk from trait A. The Aliased_Talker makes use of the as operator to be able to use B's bigTalk implementation under an additional alias talk. ]]> Using the as syntax, one can also adjust the visibility of the method in the exhibiting class. ]]> Just as classes can make use of traits, so can other traits. By using one or more traits in a trait definition, it can be composed partially or entirely of the members defined in those other traits. sayHello(); $o->sayWorld(); ?> ]]> &example.outputs; Traits support the use of abstract methods in order to impose requirements upon the exhibiting class. getWorld(); } abstract public function getWorld(); } class MyHelloWorld { private $world; use Hello; public function getWorld() { return $this->world; } public function setWorld($val) { $this->world = $val; } } ?> ]]> Static variables can be referred to in trait methods, but cannot be defined by the trait. Traits can, however, define static methods for the exhibiting class. inc(); // echo 1 $p = new C2(); $p->inc(); // echo 1 ?> ]]> ]]> Traits can also define properties. x; ?> ]]> If a trait defines a property then a class can not define a property with the same name, otherwise an error is issued. It is an E_STRICT if the class definition is compatible (same visibility and initial value) or fatal error otherwise. ]]>