Using namespaces: Basics
(PHP 5 >= 5.3.0, PHP 7, PHP 8)
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 tocurrentdirectory/foo.txtwhere 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 tocurrentdirectory/subdirectory/foo.txt. -
Absolute path name like
/main/foo.txt. This resolves to/main/foo.txt.
-
Unqualified name, or an unprefixed class name like
$a = new foo();orfoo::staticmethod();. If the current namespace iscurrentnamespace, this resolves tocurrentnamespace\foo. If the code is global, non-namespaced code, this resolves tofoo. 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();orsubnamespace\foo::staticmethod();. If the current namespace iscurrentnamespace, this resolves tocurrentnamespace\subnamespace\foo. If the code is global, non-namespaced code, this resolves tosubnamespace\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
?>
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