PHP是单继承的语言，在PHP 5.4 Traits出现之前，PHP的类无法同时从两个基类继承属性或方法。php的Traits和Go语言的组合功能有点类似，
通过在类中使用use关键字声明要组合的Trait名称，而具体某个Trait的声明使用trait关键词，Trait不能直接实例化。具体用法请看下面的代码：
<?phptrait Drive { ???public $carName = ‘BMW‘; ???public function driving() { ???????echo "driving {$this->carName}\n"; ???}}class Person { ???public function age() { ???????echo "i am 18 years old\n"; ???}}class Student extends Person { ???use Drive; ???public function study() { ???????echo "Learn to drive \n"; ???}}$student = new Student();$student->study();$student->age();$student->driving();Learn to drive i am 18 years olddriving BMW
上面的例子中，Student类通过继承Person，有了age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：
<?phptrait Drive { ???public function hello() { ???????echo "hello 周伯通\n"; ???} ???public function driving() { ???????echo "周伯通不会开车\n"; ???}}class Person { ???public function hello() { ???????echo "hello 大家好\n"; ???} ???public function driving() { ???????echo "大家都会开车\n"; ???}}class Student extends Person { ???use Drive;//trait 的方法覆盖了基类Person中的方法，所以Person中的hello 和driving被覆盖 ???public function hello() { ???????echo "hello 新学员\n";//当方法或属性同名时，当前类中的方法会覆盖 trait的 方法,所以此处hello会覆盖trait中的hello ???}}$student = new Student();$student->hello();$student->driving();hello 新学员周伯通不会开车
因此得出结论：当方法或属性同名时，当前类中的方法会覆盖 trait的 方法，而 trait 的方法又覆盖了基类中的方法。
如果要组合多个Trait，通过逗号分隔 Trait名称：
use Trait1, Trait2;
如果多个Trait中包含同名方法或者属性时，会怎样呢？答案是当组合的多个Trait包含同名属性或者方法时，需要明确声明解决冲突，否则会产生一个致命错误。
<?phptrait Trait1 { ???public function hello() { ???????echo "Trait1::hello\n"; ???} ???public function hi() { ???????echo "Trait1::hi\n"; ???}}trait Trait2 { ???public function hello() { ???????echo "Trait2::hello\n"; ???} ???public function hi() { ???????echo "Trait2::hi\n"; ???}}class Class1 { ???use Trait1, Trait2;}
//输出：Fatal error: Trait method hello has not been applied, because there are collisions with other trait methods on Class1 in使用insteadof和as操作符来解决冲突，insteadof是使用某个方法替代另一个，而as是给方法取一个别名，具体用法请看代码:
<?phptrait Trait1 { ???public function hello() { ???????echo "Trait1::hello \n"; ???} ???public function hi() { ???????echo "Trait1::hi \n"; ???}}trait Trait2 { ???public function hello() { ???????echo "Trait2::hello\n"; ???} ???public function hi() { ???????echo "Trait2::hi\n"; ???}}class Class1 { ???use Trait1, Trait2 { ???????Trait2::hello insteadof Trait1; ???????Trait1::hi insteadof Trait2; ???}}class Class2 { ???use Trait1, Trait2 { ???????Trait2::hello insteadof Trait1; ???????Trait1::hi insteadof Trait2; ???????Trait2::hi as hei; ???????Trait1::hello as hehe; ???}}$Obj1 = new Class1();$Obj1->hello();$Obj1->hi();echo "\n";$Obj2 = new Class2();$Obj2->hello();$Obj2->hi();$Obj2->hei();$Obj2->hehe();Trait2::helloTrait1::hi Trait2::helloTrait1::hi Trait2::hiTrait1::hello
as关键词还有另外一个用途，那就是修改方法的访问控制：
<?phptrait Hello { ???public function hello() { ???????echo "hello,我是周伯通\n"; ???}}class Class1 { ???use Hello { ???????hello as protected; ???}}class Class2 { ???use Hello { ???????Hello::hello as private hi; ???}}$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();$Obj2->hello(); # 输出: hello,我是周伯通,因为原来的hello方法仍然是公共的$Obj2->hi(); ?# 报致命错误，因为别名hi方法被修改成私有的Uncaught Error: Call to protected method Class1::hello() from context ‘‘ in D:\web\mytest\p.php:18
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 我是周伯通\n"; ???}}trait World { ???use Hello; ???public function sayWorld() { ???????echo "hello 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 ‘do you get World ?‘; ???}}$Obj = new HelloWorld();$Obj->sayHello();$Obj->sayWorld();echo $Obj->getWorld() . "\n";HelloWorld::doSomething();$Obj->inc();$Obj->inc();Hello 我是周伯通hello worlddo you get World ?Doing something12
PHP中的Traits用法详解
原文地址：https://www.cnblogs.com/phpper/p/8978098.html