PHP 5.6.0 released

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
?>

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User Contributed Notes 7 notes

up
50
richard at richard-sumilang dot com
6 years ago
Syntax for extending classes in namespaces is still the same.

Lets call this Object.php:

<?php

namespace com\rsumilang\common;

class
Object{
  
// ... code ...
}

?>

And now lets create a class called String that extends object in String.php:

<?php

class String extends com\rsumilang\common\Object{
  
// ... code ...
}

?>

Now if you class String was defined in the same namespace as Object then you don't have to specify a full namespace path:

<?php

namespace com\rsumilang\common;

class
String extends Object
{
  
// ... code ...
}

?>

Lastly, you can also alias a namespace name to use a shorter name for the class you are extending incase your class is in seperate namespace:

<?php

namespace com\rsumilang\util;
use
com\rsumlang\common as Common;

class
String extends Common\Object
{
  
// ... code ...
}

?>

- Richard Sumilang
up
16
Lukas Z
2 years ago
Well variables inside namespaces do not override others since variables are never affected by namespace but always global:
"Although any valid PHP code can be contained within a namespace, only four types of code are affected by namespaces: classes, interfaces, functions and constants. "

Source: "Defining Namespaces"
http://www.php.net/manual/en/language.namespaces.definition.php
up
6
philip dot preisser at arcor dot de
3 years ago
Working with variables can overwrite equal variables in other namespaces

<?php // php5 - package-version : 5.3.5-1ubuntu7.2

   
namespace
   
main
   
{}

    namespace
   
main\sub1
   
{
       
$data = 1;
    }

    namespace
   
main\sub2
   
{
        echo
$data;// 1
       
$data = 2;
    }

    namespace
   
main\sub1
   
{
        echo
$data;// 2
       
$data = 1;
    }

    namespace
    {
        echo
$data;// 1
   
}

?>
up
3
Franois Vespa
3 years ago
It seems you cannot nest a constant declaration within a if statement

<?php

namespace FOO;

if(eval)
const
BAR=true;

// will throw the following error:
// PHP Parse error:  syntax error, unexpected T_CONST

// instead use:

if(eval)
define('FOO\BAR',true);

?>
up
3
tom at tomwardrop dot com
2 years ago
It seems the file system analogy only goes so far. One thing that's missing that would be very useful is relative navigation up the namespace chain, e.g.

<?php
namespace MyProject {
   class
Person {}
}

namespace
MyProject\People {
    class
Adult extends ..\Person {}
}
?>

That would be really nice, especially if you had really deep namespaces. It would save you having to type out the full namespace just to reference a resource one level up.
up
-9
kukoman at pobox dot sk
5 years ago
PHP 5.3.0alpha2 (cli)
<?php
//  namespace MyProject\DB;
require 'db.php';

use
MyProject\DB; // fine; same as DB\
use MyProject\DB\Connection as DBC; // fine
use MyProject\DB as HM; // fine
use HM\Connection as DBC2; // class call ends with FATAL!!!

$x = new DBC(); // fine
$y = new HM\Connection(); // fine
$z = new DBC2(); // Fatal error: Class 'HM\Connection' not found
?>
up
-6
anon
6 months ago
The namespace hierarchy will normally mirror the directory hierarchy where the class files are located, but this is not a strict requirement. You could, for example, have several class flles with different namespaces in the same directory.
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