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
?>← Defining multiple namespaces in the same file
Namespaces and dynamic language features →
? 1997–2017 The PHP Documentation GroupLicensed under the Creative Commons Attribution License v3.0 or later.
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