PHP手册-__NAMESPACE__关键字（命名空间中继承其他命名空间中类注意）

常量__ NAMESPACE__的值是包含当前命名空间名称的字符串。在全局的，不包括在任何命名空间中的代码，它包含一个空的字符串。

When extending a class from another namespace that should instantiate a class from within the current namespace, you need to pass on the namespace.

<?php // File1.php
 namespace foo;
 class A {
     public function factory() {
         return new C;
     }
 }
 class C {
     public function tell() {
         echo "foo";
     }
 }
 ?>

<?php // File2.php
 namespace bar;
 class B extends \foo\A {}
 class C {
     public function tell() {
         echo "bar";
     }
 }
 ?>

<?php
 include "File1.php";
 include "File2.php";
 $b = new bar\B;
 $c = $b->factory();
 $c->tell(); // "foo" but you want "bar"
 ?>

You need to do it like this:

When extending a class from another namespace that should instantiate a class from within the current namespace, you need to pass on the namespace.

<?php // File1.php
 namespace foo;
 class A {
     protected $namespace = __NAMESPACE__;//此处是重点1，这样可以指定到当前命名空间中的对应类
     public function factory() {
         $c = $this->namespace . '\C';//此处是重点2，这样可以指定到当前命名空间中的对应类
         return new $c;
     }
 }
 class C {
     public function tell() {
         echo "foo";
     }
 }
 ?>

<?php // File2.php
 namespace bar;
 class B extends \foo\A {
     protected $namespace = __NAMESPACE__;
 }
 class C {
     public function tell() {
         echo "bar";
     }
 }
 ?>

<?php
 include "File1.php";
 include "File2.php";
 $b = new bar\B;
 $c = $b->factory();
 $c->tell(); // "bar"
 ?>

(it seems that the namespace-backslashes are stripped from the source code in the preview, maybe it works in the main view. If not: fooA was written as \foo\A and barB as bar\B)