定义命名空间

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(PHP 5 >= 5.3.0)

虽然任意合法的PHP代码都可以包含在命名空间中，但只有以下类型的代码受命名空间的影响，它们是：类（包括抽象类和traits）、接口、函数和常量。

命名空间通过关键字namespace 来声明。如果一个文件中包含命名空间，它必须在其它所有代码之前声明命名空间，除了一个以外：declare关键字。

Example #1 声明单个命名空间

  <?php
 namespace  MyProject ;

const  CONNECT_OK  =  1 ;
class  Connection  {    }
function  connect () {     }

 ?>

在声明命名空间之前唯一合法的代码是用于定义源文件编码方式的 declare 语句。另外，所有非 PHP 代码包括空白符都不能出现在命名空间的声明之前：

Example #2 声明单个命名空间

 
<html>
<?php
 namespace  MyProject ;  // 致命错误 -　命名空间必须是程序脚本的第一条语句
 ?>

另外，与PHP其它的语言特征不同，同一个命名空间可以定义在多个文件中，即允许将同一个命名空间的内容分割存放在不同的文件中。

用户评论:

[#1] kuzawinski dot marcin at NOSPAM dot gmail dot com [2014-08-05 21:52:02]

If your code looks like this:

<?php namespace NS; ?>

...and you still get "Namespace declaration statement has to be the very first statement in the script" Fatal error, then you probably use UTF-8 encoding (which is good) with Byte Order Mark, aka BOM (which is bad). Try to convert your files to "UTF-8 without BOM", and it should be ok.

[#2] Roadowl [2014-02-24 23:42:00]

quote:
Defining namespaces

(...)
Namespaces are declared using the namespace keyword. A file containing a namespace must declare the namespace at the top of the file before any other code - with one exception: the declare keyword.

end quote.

So we have a title that talks 'defining' and a piece of text that talks 'declare' three times, one of which could be referring to some other 'declare' than the former two.

Please, documentation authors -- get your act together, finally.

[#3] FatBat [2013-08-08 21:46:33]

Expanding on @danbettles note, it is better to always be explicit about which constant to use.

<?php namespace NS; define(__NAMESPACE__ .'\foo','111'); define('foo','222'); echo foo; // 111. echo \foo; // 222. echo \NS\foo // 111. echo NS\foo // fatal error. assumes \NS\NS\foo. ?>

[#4] huskyr at gmail dot com [2009-10-05 04:20:48]

"A file containing a namespace must declare the namespace at the top of the file before any other code"

It might be obvious, but this means that you *can* include comments and white spaces before the namespace keyword.

<?php // Lots // of // interesting // comments and white space namespace Foo; class Bar { } ?>

[#5] jeremeamia at gmail dot com [2009-07-14 08:43:55]

You should not try to create namespaces that use PHP keywords. These will cause parse errors.

Examples:

<?php namespace Project/Classes/Function; // Causes parse errors namespace Project/Abstract/Factory; // Causes parse errors ?>

[#6] danbettles at yahoo dot co dot uk [2009-04-14 12:02:21]

Regarding constants defined with define() inside namespaces...

define() will define constants exactly as specified. So, if you want to define a constant in a namespace, you will need to specify the namespace in your call to define(), even if you're calling define() from within a namespace. The following examples will make it clear.

The following code will define the constant "MESSAGE" in the global namespace (i.e. "\MESSAGE").

<?php namespace test; define('MESSAGE', 'Hello world!'); ?>

The following code will define two constants in the "test" namespace.

<?php namespace test; define('test\HELLO', 'Hello world!'); define(__NAMESPACE__ . '\GOODBYE', 'Goodbye cruel world!'); ?>

[#7] David Drakard [2008-09-07 05:56:02]

I agree with SR, the new namespaces feature has solved a number of problems for me which would have required horrible coding to solve otherwise.

An example use:
Say you are making a small script, and write a class to connect to a database, calling it 'connection'. If you find your script useful and gradually expand it into a large application, you may want to rename the class. Without namespaces, you have to change the name and every reference to it (say in inheriting objects), possibly creating a load of bugs. With namespaces you can drop the related classes into a namespace with one line of code, and less chance of errors.

This is by no means one of the biggest problems namespaces solve; I would suggest reading about their advantages before citicising them. They provide an elegant solutions to several problems involved in creating complex systems.

[#8] Baptiste [2008-05-14 12:47:03]

There is nothing wrong with PHP namespaces, except that those 2 instructions give a false impression of package management.
... while they just correspond to the "with()" instruction of Javascript.

By contrast, a package is a namespace for its members, but it offers more (like deployment facilities), and a compiler knows exactly what classes are in a package, and where to find them.

[#9] Anonymous [2008-04-01 11:11:34]

@ RS: Also, you can specify how your __autoload() function looks for the files. That way another users namespace classes cannot overwrite yours unless they replace your file specifically.