Using namespaces: Basics

(PHP 5 >= 5.3.0, PHP 7)

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:

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.
Relative path name like subdirectory/foo.txt. This resolves to currentdirectory/subdirectory/foo.txt.
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:

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.
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.
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
?>

User Contributed Notes

richard at richard-sumilang dot com

8 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

Anonymous

1 year ago


<?php

namespace Foo;

try {
    // Something awful here
    // That will throw a new exception from SPL
} 
catch (Exception as $ex) {
    // We will never get here
    // This is because we are catchin Foo\Exception
}
?>

Instead use fully qualified name for the exception to catch it

<?php 

namespace Foo;

try {
    // something awful here
    // That will throw a new exception from SPL
} 
catch (\Exception as $ex) {
    // Now we can get here at last
}
?>

Lukas Z

4 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

tom at tomwardrop dot com

4 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.

philip dot preisser at arcor dot de

4 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
    }

?>

Franois Vespa

5 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);

?>

anon

2 years 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.

kukoman at pobox dot sk

7 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

?>