PHP Referencing a class or function in a namespace


As shown in Declaring Namespaces, we can define a class in a namespace as follows:

namespace MyProject\Shapes;

class Rectangle { ... }

To reference this class the full path (including the namespace) needs to be used:

$rectangle = new MyProject\Shapes\Rectangle();

This can be shortened by importing the class via the use-statement:

// Rectangle becomes an alias to MyProject\Shapes\Rectangle
use MyProject\Shapes\Rectangle;

$rectangle = new Rectangle();

As for PHP 7.0 you can group various use-statements in one single statement using brackets:

use MyProject\Shapes\{
    Rectangle,         //Same as `use MyProject\Shapes\Rectangle`
    Circle,            //Same as `use MyProject\Shapes\Circle`
    Triangle,          //Same as `use MyProject\Shapes\Triangle`
    Polygon\FiveSides, //You can also import sub-namespaces
    Polygon\SixSides   //In a grouped `use`-statement

$rectangle = new Rectangle();

Sometimes two classes have the same name. This is not a problem if they are in a different namespace, but it could become a problem when attempting to import them with the use-statement:

use MyProject\Shapes\Oval;
use MyProject\Languages\Oval; // Apparantly Oval is also a language!
// Error!

This can be solved by defining a name for the alias yourself using the as keyword:

use MyProject\Shapes\Oval as OvalShape;
use MyProject\Languages\Oval as OvalLanguage;

To reference a class outside the current namespace, it has to be escaped with a \, otherwise a relative namespace path is assumed from the current namespace:

namespace MyProject\Shapes;

// References MyProject\Shapes\Rectangle. Correct!
$a = new Rectangle();

// References MyProject\Shapes\Rectangle. Correct, but unneeded!
$a = new \MyProject\Shapes\Rectangle(); 

// References MyProject\Shapes\MyProject\Shapes\Rectangle. Incorrect!
$a = new MyProject\Shapes\Rectangle(); 

// Referencing StdClass from within a namespace requires a \ prefix
// since it is not defined in a namespace, meaning it is global.

// References StdClass. Correct!
$a = new \StdClass(); 

// References MyProject\Shapes\StdClass. Incorrect!
$a = new StdClass();