Although any valid PHP code can be contained within a namespace, only three type of code are affected by namespaces: classes, 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.
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