namespace keyword and __NAMESPACE__ constant
Something that is not immediately obvious, particular with PHP 5.3, is that namespace resolutions within an import are not resolved recursively. i.e.: if you alias an import and then use that alias in another import then this latter import will not be fully resolved with the former import.
For example:
use \Controllers as C;
use C\First;
use C\Last;
Both the First and Last namespaces are NOT resolved as \Controllers\First or \Controllers\Last as one might intend.
Using namespaces: Aliasing/Importing
(PHP 5 >= 5.3.0)
The ability to refer to an external fully qualified name with an alias, or importing, is an important feature of namespaces. This is similar to the ability of unix-based filesystems to create symbolic links to a file or to a directory.
PHP namespaces support three kinds of aliasing or importing: aliasing a class name, aliasing an interface name, and aliasing a namespace name. Note that importing a function or constant is not supported.
In PHP, aliasing is accomplished with the use operator. Here is an example showing all 3 kinds of importing:
Example #1 importing/aliasing with the use operator
<?php
namespace foo;
use My\Full\Classname as Another;
// this is the same as use My\Full\NSname as NSname
use My\Full\NSname;
// importing a global class
use ArrayObject;
$obj = new namespace\Another; // instantiates object of class foo\Another
$obj = new Another; // instantiates object of class My\Full\Classname
NSname\subns\func(); // calls function My\Full\NSname\subns\func
$a = new ArrayObject(array(1)); // instantiates object of class ArrayObject
// without the "use ArrayObject" we would instantiate an object of class foo\ArrayObject
?>
PHP additionally supports a convenience shortcut to place multiple use statements on the same line
Example #2 importing/aliasing with the use operator, multiple use statements combined
<?php
use My\Full\Classname as Another, My\Full\NSname;
$obj = new Another; // instantiates object of class My\Full\Classname
NSname\subns\func(); // calls function My\Full\NSname\subns\func
?>
Importing is performed at compile-time, and so does not affect dynamic class, function or constant names.
Example #3 Importing and dynamic names
<?php
use My\Full\Classname as Another, My\Full\NSname;
$obj = new Another; // instantiates object of class My\Full\Classname
$a = 'Another';
$obj = new $a; // instantiates object of class Another
?>
In addition, importing only affects unqualified and qualified names. Fully qualified names are absolute, and unaffected by imports.
Example #4 Importing and fully qualified names
<?php
use My\Full\Classname as Another, My\Full\NSname;
$obj = new Another; // instantiates object of class My\Full\Classname
$obj = new \Another; // instantiates object of class Another
$obj = new Another\thing; // instantiates object of class My\Full\Classname\thing
$obj = new \Another\thing; // instantiates object of class Another\thing
?>
Scoping rules for importing
The use keyword must be declared in the outermost scope of a file (the global scope) or inside namespace declarations. This is because the importing is done at compile time and not runtime, so it cannot be block scoped. The following example will show an illegal use of the use keyword:
Example #5 Illegal importing rule
<?php
namespace Languages;
class Greenlandic
{
use Languages\Danish;
...
}
?>
Note:
Importing rules are per file basis, meaning included files will NOT inherit the parent file's importing rules.
add a note
User Contributed Notes
Using namespaces: Aliasing/Importing - [9 notes]
cl ¶
3 months ago
x at d dot a dot r dot k dot REMOVEDOTSANDTHIS dot gray dot org ¶
6 months ago
You are allowed to "use" the same resource multiple times as long as it is imported under a different alias at each invocation.
For example:
<?php
use Lend;
use Lend\l1;
use Lend\l1 as l3;
use Lend\l2;
use Lend\l1\Keller;
use Lend\l1\Keller as Stellar;
use Lend\l1\Keller as Zellar;
use Lend\l2\Keller as Dellar;
...
?>
In the above example, "Keller", "Stellar", and "Zellar" are all references to "\Lend\l1\Keller", as are "Lend\l1\Keller", "l1\Keller", and "l3\Keller".
k at webnfo dot com ¶
2 months ago
Note that you can not alias global namespace:
use \ as test;
echo test\strlen('');
won't work.
nsdhami at live dot jp ¶
2 years ago
The "use" keyword can not be declared inside the function or method. It should be declared as global, after the "namespace" as:
<?php
namespace mydir;
// works perfectly
use mydir/subdir/Class1 as Class1;
function fun1()
{
// Parse error: syntax error, unexpected T_USE
use mydir/subdir/Class1 as Class1;
}
class Class2
{
public function fun2()
{
// Parse error: syntax error, unexpected T_USE
use mydir/subdir/Class1 as Class1;
}
}
?>
c dot 1 at smithies dot org ¶
1 year ago
If you are testing your code at the CLI, note that namespace aliases do not work!
(Before I go on, all the backslashes in this example are changed to percent signs because I cannot get sensible results to display in the posting preview otherwise. Please mentally translate all percent signs henceforth as backslashes.)
Suppose you have a class you want to test in myclass.php:
<?php
namespace my%space;
class myclass {
// ...
}
?>
and you then go into the CLI to test it. You would like to think that this would work, as you type it line by line:
require 'myclass.php';
use my%space%myclass; // should set 'myclass' as alias for 'my%space%myclass'
$x = new myclass; // FATAL ERROR
I believe that this is because aliases are only resolved at compile time, whereas the CLI simply evaluates statements; so use statements are ineffective in the CLI.
If you put your test code into test.php:
<?php
require 'myclass.php';
use my%space%myclass;
$x = new myclass;
//...
?>
it will work fine.
I hope this reduces the number of prematurely bald people.
samuel dot roze at gmail dot com ¶
1 year ago
(All the backslashes in namespaces are slashes because I can't figure out how to post backslashes here.)
You can have the same "use" for a class and a namespace. For example, if you have these files:
<?php
// foo/bar.php
namespace foo;
class bar
{
public function __toString ()
{
return 'foo\bar\__toString()';
}
}
?>
<?php
// foo/bar/MyClass.php
namespace foo/bar;
class MyClass
{
public function __toString ()
{
return 'foo\bar\MyClass\__toString()';
}
}
?>
In another namespace, you can do:
<?php
namespace another;
require_once 'foo/bar.php';
require_once 'foo/bar/MyClass.php';
use foo/bar;
$bar = new bar();
echo $bar."\n";
$class = new bar/MyClass();
echo $class."\n";
?>
And it will makes the following output:
foo\bar\__toString()
foo\bar\MyClass\__toString()
Anonymous ¶
2 months ago
The last example on this page shows a possibly incorrect attempt of aliasing, but it is totally correct to import a trait \Languages\Languages\Danish.
thinice at gmail.com ¶
2 years ago
Because imports happen at compile time, there's no polymorphism potential by embedding the use keyword in a conditonal.
e.g.:
<?php
if ($objType == 'canine') {
use Animal\Canine as Beast;
}
if ($objType == 'bovine') {
use Animal\Bovine as Beast;
}
$oBeast = new Beast;
$oBeast->feed();
?>