# Using namespaces: Basics

Domains:

(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:

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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