<?php    trait Drive {        public $carName = 'trait';        public function driving() {            echo "driving {$this->carName}\n";        }    }    class Person {        public function eat() {            echo "eat\n";        }    }    class Student extends Person {        use Drive;        public function study() {            echo "study\n";        }    }    $student = new Student();    $student->study();    $student->eat();    $student->driving();

输出结果如下：

studyeatdriving trait

上面的例子中，Student类通过继承Person，有了eat方法，通过组合Drive，有了driving方法和属性carName。

如果Trait、基类和本类中都存在某个同名的属性或者方法，最终会保留哪一个呢？通过下面的代码测试一下：

<?php     trait Drive {        public function hello() {            echo "hello drive\n";        }        public function driving() {            echo "driving from drive\n";        }    }    class Person {        public function hello() {            echo "hello person\n";        }        public function driving() {            echo "driving from person\n";        }    }    class Student extends Person {        use Drive;        public function hello() {            echo "hello student\n";        }    }    $student = new Student();    $student->hello();    $student->driving();

输出结果如下：

hello studentdriving from drive

因此得出结论：当方法或属性同名时，当前类中的方法会覆盖 trait的 方法，而 trait 的方法又覆盖了基类中的方法。

如果要组合多个Trait，通过逗号分隔 Trait名称：

use Trait1, Trait2;

如果多个Trait中包含同名方法或者属性时，会怎样呢？答案是当组合的多个Trait包含同名属性或者方法时，需要明确声明解决冲突，否则会产生一个致命错误。

<?phptrait Trait1 {    public function hello() {        echo "Trait1::hello\n";    }    public function hi() {        echo "Trait1::hi\n";    }}trait Trait2 {    public function hello() {        echo "Trait2::hello\n";    }    public function hi() {        echo "Trait2::hi\n";    }}class Class1 {    use Trait1, Trait2;}

输出结果如下：

PHP Fatal error:  Trait method hello has not been applied, because there are collisions with other trait methods on Class1 in ~/php54/trait_3.php on line 20

使用insteadof和as操作符来解决冲突，insteadof是使用某个方法替代另一个，而as是给方法取一个别名，具体用法请看代码：

<?phptrait Trait1 {    public function hello() {        echo "Trait1::hello\n";    }    public function hi() {        echo "Trait1::hi\n";    }}trait Trait2 {    public function hello() {        echo "Trait2::hello\n";    }    public function hi() {        echo "Trait2::hi\n";    }}class Class1 {    use Trait1, Trait2 {        Trait2::hello insteadof Trait1;        Trait1::hi insteadof Trait2;    }}class Class2 {    use Trait1, Trait2 {        Trait2::hello insteadof Trait1;        Trait1::hi insteadof Trait2;        Trait2::hi as hei;        Trait1::hello as hehe;    }}$Obj1 = new Class1();$Obj1->hello();$Obj1->hi();echo "\n";$Obj2 = new Class2();$Obj2->hello();$Obj2->hi();$Obj2->hei();$Obj2->hehe();

输出结果如下：

Trait2::helloTrait1::hiTrait2::helloTrait1::hiTrait2::hiTrait1::hello

as关键词还有另外一个用途，那就是修改方法的访问控制：

<?php    trait Hello {        public function hello() {            echo "hello,trait\n";        }    }    class Class1 {        use Hello {            hello as protected;        }    }    class Class2 {        use Hello {            Hello::hello as private hi;        }    }    $Obj1 = new Class1();    $Obj1->hello(); # 报致命错误，因为hello方法被修改成受保护的    $Obj2 = new Class2();    $Obj2->hello(); # 原来的hello方法仍然是公共的    $Obj2->hi();  # 报致命错误，因为别名hi方法被修改成私有的

Trait 也能组合Trait，Trait中支持抽象方法、静态属性及静态方法，测试代码如下：

<?phptrait Hello {    public function sayHello() {        echo "Hello\n";    }}trait World {    use Hello;    public function sayWorld() {        echo "World\n";    }    abstract public function getWorld();    public function inc() {        static $c = 0;        $c = $c + 1;        echo "$c\n";    }    public static function doSomething() {        echo "Doing something\n";    }}class HelloWorld {    use World;    public function getWorld() {        return 'get World';    }}$Obj = new HelloWorld();$Obj->sayHello();$Obj->sayWorld();echo $Obj->getWorld() . "\n";HelloWorld::doSomething();$Obj->inc();$Obj->inc();

输出结果如下：

HelloWorldget WorldDoing something12