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();