在同一个文件中定义多个命名空间

(PHP 5 >= 5.3.0, PHP 7)

也可以在同一个文件中定义多个命名空间。在同一个文件中定义多个命名空间有两种语法形式。

Example #1 定义多个命名空间，简单组合语法

<?php
namespace MyProject;
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }
namespace AnotherProject;
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }
?>

不建议使用这种语法在单个文件中定义多个命名空间。建议使用下面的大括号形式的语法。

Example #2 定义多个命名空间，大括号语法

<?php
namespace MyProject {
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }
}
namespace AnotherProject {
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }
}
?>

在实际的编程实践中，非常不提倡在同一个文件中定义多个命名空间。这种方式的主要用于将多个 PHP 脚本合并在同一个文件中。

将全局的非命名空间中的代码与命名空间中的代码组合在一起，只能使用大括号形式的语法。全局代码必须用一个不带名称的 namespace 语句加上大括号括起来，例如：

Example #3 定义多个命名空间和不包含在命名空间中的代码

<?php
namespace MyProject {
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }
}
namespace { // global code
session_start();
$a = MyProject\connect();
echo MyProject\Connection::start();
}
?>

除了开始的declare语句外，命名空间的括号外不得有任何PHP代码。

Example #4 定义多个命名空间和不包含在命名空间中的代码

<?php
declare(encoding='UTF-8');
namespace MyProject {
const CONNECT_OK = 1;
class Connection { /* ... */ }
function connect() { /* ... */  }
}
namespace { // 全局代码
session_start();
$a = MyProject\connect();
echo MyProject\Connection::start();
}
?>

using of global namespaces and multiple namespaces in one PHP file increase the complexity and decrease readability of the code.
Let's try not use this scheme even it's very necessary (although there is not)

<?php
// You cannot mix bracketed namespace declarations with unbracketed namespace declarations - will result in a Fatal error
namespace a;
echo "I belong to namespace a";
namespace b {
  echo "I'm from namespace b";
}

<?php
//Namespace can be used in this way also
namespace MyProject {
function connect() { echo "ONE"; }
  Sub\Level\connect();
}
namespace MyProject\Sub {
  
function connect() { echo "TWO"; }
  Level\connect();
}
namespace MyProject\Sub\Level {
  
  function connect() { echo "THREE"; }  
  \MyProject\Sub\Level\connect(); // OR we can use this as below
  connect();
}

If you have the habit to always use the closing PHP tag "?>" in your test files, remember that with the bracketed syntax code outside the brackets, including new lines outside the PHP tags, is not allowed. In particular, even though PHP sees a new line after the closing tag as a part of the line and eats it, some editors, such as Gedit, Gvim, Vim and Nano in Ubuntu, will add yet another new line after this new line and this will create an error.

//call same named function using namespace
//food.php
<?php
namespace Food;
require ('Apple.php');
require('Orange.php');
use Apples;
use Oranges;
 Apples\eat();
 Oranges\eat();
 ?>
//Apple.php
<?php
namespace Apples;
function eat()
{
 echo "eat apple";
}
?>
//Orange.php
<?php
namespace Oranges;
function eat()
{
 echo "eat Orange";
}
?>

Notice it's not allowed to mix bracketed namespace with unbracketed namespace declarations.
<?php
namespace MyTest;
const TEST = 777;
//in this way
echo \MyTest\TEST . '<br />';
//Or this way
echo TEST . '<br />';
namespace AnotherTest {
const TEST = 555;
echo \AnotherTest\TEST . '<br />';
echo \MyTest\TEST;
}
?>
This code will issue a fatal error, like this: Fatal error: Cannot mix bracketed namespace declarations with unbracketed namespace declarations in C:\xampp\htdocs\teste2.php on line 11.

There are rational examples of where the ability to blend multiple namespaces into a single file is not only desirable but also absolutely necessary. An example of where this ability is useful is over in the very popular phpseclib library where they are PSR-4 compliant but, in order to be compliant, they have to read a directory of files to know what classes are available so that the autoloader can load the correct files. If they, instead, just bundled the defaults into one file using this mechanism already supported by PHP core, there would be no need to do extraneous scanning of the file system.
That's just one legitimate use-case where strict compliance with PSRs gets in the way of good software development.

Be careful with include combined to namespaces:
file b.php
<?php
  const WHERE_I_AM = 'I am in B';
  
  function i_am_in() { 
    \A\cr_echo(WHERE_I_AM);
  }
?>
file c.php
<?php
  namespace C {
    const WHERE_I_AM = 'I am in C';
    
    function i_am_in() { 
      \A\cr_echo(WHERE_I_AM);
    }
  }
?>
main file
<?php
  
  namespace A {
  
    const CR = "\r\n";
    const WHERE_I_AM = 'I am in A';
    
    function cr_echo($msg) { 
      echo $msg . CR;
    }
    
    function i_am_in() { 
      cr_echo(WHERE_I_AM);
    }
  }
  
  namespace B {
    require 'b.php';
  }
  
  namespace {
    require 'c.php';
    
    \A\i_am_in(); //ok
    \B\i_am_in(); // fatal-error
    \C\i_am_in(); //ok
  }
?>

With the bracketed syntax, a simple white space after the closing "?>" of the file, even a new line that is some times considered as the end of a line and thus a part of the line, will (at the least in some php installations) be considered as code outside the brackets and will result in an error. It's general good practice to avoid white space after the closing "?>". Just pointing out that it is another case where it matters.

In addition to kothnok at gmail dot com
When using namespaces with brackets, you should define the USE statements within the brackets. His example sets the global use for the file.
<?php
namespace Space1;
use SomeNamespace\FunctionCollection as FC;
{
   FC\hello_world();
}
namespace Space2;
{
   // This can be used here
   FC\hello_world();
}
?>
While the preferred method with brackets is this:
<?php
namespace Space1
{
   use SomeNamespace\FunctionCollection as FC;
   FC\hello_world();
}
namespace Space2;
{
   // This will not work now
   FC\hello_world();
}
?>