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Defining namespaces

(PHP 5 >= 5.3.0)

Although any valid PHP code can be contained within a namespace, only the following types of code are affected by namespaces: classes (including abstracts and traits), interfaces, functions and constants.

Namespaces are declared using the namespace keyword. A file containing a namespace must declare the namespace at the top of the file before any other code - with one exception: the declare keyword.

Example #1 Declaring a single namespace

<?php
namespace MyProject;

const 
CONNECT_OK 1;
class 
Connection /* ... */ }
function 
connect() { /* ... */  }

?>
The only code construct allowed before a namespace declaration is the declare statement, for defining encoding of a source file. In addition, no non-PHP code may precede a namespace declaration, including extra whitespace:

Example #2 Declaring a single namespace

<html>
<?php
namespace MyProject// fatal error - namespace must be the first statement in the script
?>

In addition, unlike any other PHP construct, the same namespace may be defined in multiple files, allowing splitting up of a namespace's contents across the filesystem.

PHP Manual