PHP | Namespace

Like C++, PHP Namespaces are the way of encapsulating items so that same names can be reused without name conflicts.

• It can be seen as an abstract concept in many places. It allows redeclaring the same functions/classes/interfaces/constant functions in the separate namespace without getting the fatal error.
• A namespace is a hierarchically labeled code block holding a regular PHP code.
• A namespace can contain valid PHP code.
• Namespace affects following types of code: classes (including abstracts and traits), interfaces, functions, and constants.
• Namespaces are declared using the namespace keyword.

A namespace must be declared the namespace at the top of the file before any other code – with one exception: the declare keyword. 

php

<?php
namespace MyNamespaceName {

    // Regular PHP code
    function hello()
    {
        echo 'Hello I am Running from a namespace!';
    }
}
?>

If namespace is declared globally, then declare it without any name. 

php

<?php
namespace {

    // Global space!
}
?>

Multiple namespaces can be declared within a single PHP code. 

php

<?php
namespace MyNamespace1 {

}

namespace MyNamespace2 {

}

namespace {

}
?>

A namespace is used to avoid conflicting definitions and introduce more flexibility and organization in the code base. Just like directories, namespace can contain a hierarchy know as subnamespaces. PHP uses the backslash as its namespace separator.
Example:

php

<?php
namespace MyNamespaceName;
function hello()
    {
        echo 'Hello I am Running from a namespace!';
    }

// Resolves to MyNamespaceName\hello
hello();

// Explicitly resolves to MyNamespaceName\hello
namespace\hello();
?>

Aliasing in Namespaces

Importing is achieved by using the ‘use’ keyword. Optionally, It can specify a custom alias with the ‘as’ keyword. 
Example:

php

<?php
namespace MyNamespaceName;

require 'project/database/connection.php';

use Project\Database\Connection as Connection;

$connection = new Connection();

use Project\Database as Database;

$connection = new Database\Connection();
?>

It is possible to dynamically call namespaced code, dynamic importing is not supported. 

php

<?php
namespace OtherProject;

$title = 'geeks';

// This is valid PHP
require 'project/blog/title/' . $title . '.php';

// This is not
use Project\Blog\title\$title;
?>

Reference : http://php.net/manual/en/language.namespaces.php