定义命名空间

(PHP 5 >= 5.3.0, PHP 7)

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。

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.

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!');
?>

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.
?>

"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 {
}
?>

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
?>

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.

@ 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.

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.

namespace declare must be starting of the script, only php tag can be written before namespace like this
<?php
namespace test;    //this is right way
class Testclass{/*****/}
function test(/****/){}
?>
if you used any other code like html tag or any javascript etc then it gives fatal error.
Example:
<html>
<head></head>
<body>
<?php
namespace test;    //Fatal Error
class Testclass{/*****/}
function test(/****/){}
?>

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.