php命名空间的设计思想和缺点
相比C#等语言，你可以在php函数里面随意定义变量并赋值，而不用担心覆盖了全局变量，或者类变量；你也可以随意的定义类变量，而不用担心会和函数名冲突，因为变量前面都有个$。
php的命名空间和全局变量、局部变量的设计思想一样，都是为了尽可能的减少命名冲突。php的命名空间可以和文件系统类似，有相对命名空间和绝对命名空间。
定义命名空间
必须是第一个语句，前面不能有任何语句，注释可以。
必须是绝对命名空间。
namespace bookrpg\config;
//或者
namespace bookrpg\config
{
}
namespace \bookrpg\config; //错误，不能有前导\12345678910导入命名空间
必须是绝对命名空间。
use bookrpg\config\ClassName; use \bookrpg\config\ClassName; //可以有前导\，但不建议加 use bookrpg\config\ClassName as NewClassName; use Exception; use \Exception; //可以有前导\，但不建议加123456
使用命名空间
使用绝对命名空间
$cls = new \bookrpg\config\ClassName();
class MyException extends \Exception
{
}123456使用相对命名空间
namespace bookrpg
{
//等同于new \bookrpg\config\ClassName();
$cls = new config\ClassName();
//错误，php会认为是\bookrpg\Exception，而不是全局类Exception
$ex = new Exception();
}123456789怪异的地方
C++、Java、C#都有相似的命名空间概念，而php的命名空间不跟随大流，有自己的一套独立的思想，对于先入为主的人来来说就显得怪异和令人迷惑。
命名空间中居然有反斜杠？
点号(.)已经被用作字符串连接符了，又要区分绝对和相对命名空间，new .a.b.c()太不美观，所以选择反斜杠吧。
一旦使用了命名空间，使用全局类也要加上“\”或者use一下，累不累呀？
没办法，为了严格类比文件系统的概念，也为了随便定义类名，而不用担心类名冲突。
既然是严格类比文件系统，为什么定义和导入命名空间的时候，前面不加\呢？
只能说，php的设计者不是完美主义者。
但php的命名空间确实缺乏逻辑完美性：
前导\的使用不统一，同样是绝对命名空间，定义的时候不允许用，导入的时候却允许用。
即使是全局类，在非全局命名空间下，也必须使用前导\，而全局函数和变量又不受此限制。
使用建议
定义和导入命名空间的时候，统一不加前导\
所有的类都使用use导入，包括全局类，只命名空间类，有冲突的就as一下
禁止直接使用相对命名空间
理解了php命名空间的设计思想，并避免其设计不完美的地方，这样看来php的命名空间也是很容易接受的。
上面是我整理给大家的php命名空间的设计思想和缺点，希望今后会对大家有帮助。
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