Defining multiple namespaces in the same file
Namespaces and dynamic language features
Namespaces
PHP Manual

## Using namespaces: Basics

Before discussing the use of namespaces, it is important to understand how PHP knows which namespaced element your code is requesting. A simple analogy can be made between PHP namespaces and a filesystem. There are three ways to access a file in a file system:

1. Relative file name like foo.txt. This resolves to currentdirectory/foo.txt where currentdirectory is the directory currently occupied. So if the current directory is /home/foo, the name resolves to /home/foo/foo.txt.
2. Relative path name like subdirectory/foo.txt. This resolves to currentdirectory/subdirectory/foo.txt.
3. Absolute path name like /main/foo.txt. This resolves to /main/foo.txt.

The same principle can be applied to namespaced elements in PHP. For example, a class name can be referred to in three ways:

1. Unqualified name, or an unprefixed class name like $a = new foo(); or foo::staticmethod();. If the current namespace is currentnamespace, this resolves to currentnamespace\foo. If the code is global, non-namespaced code, this resolves to foo. One caveat: unqualified names for functions and constants will resolve to global functions and constants if the namespaced function or constant is not defined. See Using namespaces: fallback to global function/constant for details.
2. Qualified name, or a prefixed class name like $a = new subnamespace\foo(); or subnamespace\foo::staticmethod();. If the current namespace is currentnamespace, this resolves to currentnamespace\subnamespace\foo. If the code is global, non-namespaced code, this resolves to subnamespace\foo.
3. Fully qualified name, or a prefixed name with global prefix operator like $a = new \currentnamespace\foo(); or \currentnamespace\foo::staticmethod();. This always resolves to the literal name specified in the code, currentnamespace\foo.

Here is an example of the three kinds of syntax in actual code:

file1.php

`<?php
namespace Foo\Bar\subnamespace;
const FOO = 1;
function foo() {}
class foo
{
static function staticmethod() {}
}
?>`

file2.php

`<?php
namespace Foo\Bar;
include 'file1.php';
const FOO = 2;
function foo() {}
class foo
{
static function staticmethod() {}
}
/* Unqualified name */
foo(); // resolves to function Foo\Bar\foo
foo::staticmethod(); // resolves to class Foo\Bar\foo, method staticmethod
echo FOO; // resolves to constant Foo\Bar\FOO
/* Qualified name */
subnamespace\foo(); // resolves to function Foo\Bar\subnamespace\foo
subnamespace\foo::staticmethod(); // resolves to class Foo\Bar\subnamespace\foo,
// method staticmethod
echo subnamespace\FOO; // resolves to constant Foo\Bar\subnamespace\FOO
/* Fully qualified name */
\Foo\Bar\foo(); // resolves to function Foo\Bar\foo
\Foo\Bar\foo::staticmethod(); // resolves to class Foo\Bar\foo, method staticmethod
echo \Foo\Bar\FOO; // resolves to constant Foo\Bar\FOO
?>`

Note that to access any global class, function or constant, a fully qualified name can be used, such as \strlen() or \Exception or \INI_ALL.

Example #1 Accessing global classes, functions and constants from within a namespace

`<?php
namespace Foo;
function strlen() {}
const INI_ALL = 3;
class Exception {}
$a = \strlen('hi'); // calls global function strlen
$b = \INI_ALL; // accesses global constant INI_ALL
$c = new \Exception('error'); // instantiates global class Exception
?>`

Defining multiple namespaces in the same file
Namespaces and dynamic language features
Namespaces
PHP Manual