PHP Australia Conference 2015

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.

All versions of PHP that support namespaces support three kinds of aliasing or importing: aliasing a class name, aliasing an interface name, and aliasing a namespace name. PHP 5.6+ also allows aliasing or importing function and constant names.

In PHP, aliasing is accomplished with the use operator. Here is an example showing all 5 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;

// importing a function (PHP 5.6+)
use function My\Full\functionName;

// aliasing a function (PHP 5.6+)
use function My\Full\functionName as func;

// importing a constant (PHP 5.6+)
use const My\Full\CONSTANT;

$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
func(); // calls function My\Full\functionName
echo CONSTANT// echoes the value of My\Full\CONSTANT
?>
Note that for namespaced names (fully qualified namespace names containing namespace separator, such as Foo\Bar as opposed to global names that do not, such as FooBar), the leading backslash is unnecessary and not recommended, as import names must be fully qualified, and are not processed relative to the current namespace.

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 AnotherMy\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 AnotherMy\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 AnotherMy\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;

    ...
}
?>

Notă:

Importing rules are per file basis, meaning included files will NOT inherit the parent file's importing rules.

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User Contributed Notes 10 notes

up
14
x at d dot a dot r dot k dot REMOVEDOTSANDTHIS dot gray dot org
1 year 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".
up
11
k at webnfo dot com
1 year ago
Note that you can not alias global namespace:

use \ as test;

echo test\strlen('');

won't work.
up
11
c dot 1 at smithies dot org
3 years 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.
up
6
samuel dot roze at gmail dot com
2 years 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()
up
2
anon
8 months ago
The <?php use ?> statement does not load the class file. You have to do this with the <?php require ?> statement or by using an autoload function.
up
3
cl
1 year ago
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.
up
1
thinice at gmail.com
3 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();
?>
up
0
Anonymous
1 year 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.
up
-5
Jan Tvrdk
3 years ago
Importing and aliasing an interface name is also supported.
up
-6
nsdhami at live dot jp
4 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;
    }
}
?>
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