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Defining namespaces

(PHP 5 >= 5.3.0, PHP 7)

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.

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User Contributed Notes 10 notes

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142
kuzawinski dot marcin at NOSPAM dot gmail dot com
5 years ago
If your code looks like this:

<?php
   
namespace NS;
?>

...and you still get "Namespace declaration statement has to be the very first statement in the script" Fatal error, then you probably use UTF-8 encoding (which is good) with Byte Order Mark, aka BOM (which is bad). Try to convert your files to "UTF-8 without BOM", and it should be ok.
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117
danbettles at yahoo dot co dot uk
10 years ago
Regarding constants defined with define() inside namespaces...

define() will define constants exactly as specified.  So, if you want to define a constant in a namespace, you will need to specify the namespace in your call to define(), even if you're calling define() from within a namespace.  The following examples will make it clear.

The following code will define the constant "MESSAGE" in the global namespace (i.e. "\MESSAGE").

<?php
namespace test;
define('MESSAGE', 'Hello world!');
?>

The following code will define two constants in the "test" namespace.

<?php
namespace test;
define('test\HELLO', 'Hello world!');
define(__NAMESPACE__ . '\GOODBYE', 'Goodbye cruel world!');
?>
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73
FatBat
6 years ago
Expanding on @danbettles note, it is better to always be explicit about which constant to use.

<?php
   
namespace NS;

   
define(__NAMESPACE__ .'\foo','111');
   
define('foo','222');

    echo
foo// 111.
   
echo \foo// 222.
   
echo \NS\foo  // 111.
   
echo NS\foo  // fatal error. assumes \NS\NS\foo.
?>
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61
huskyr at gmail dot com
10 years ago
"A file containing a namespace must declare the namespace at the top of the file before any other code"

It might be obvious, but this means that you *can* include comments and white spaces before the namespace keyword.

<?php
// Lots
// of
// interesting
// comments and white space

namespace Foo;
class
Bar {
}
?>
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51
jeremeamia at gmail dot com
10 years ago
You should not try to create namespaces that use PHP keywords. These will cause parse errors.

Examples:

<?php
namespace Project/Classes/Function; // Causes parse errors
namespace Project/Abstract/Factory; // Causes parse errors
?>
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8
Baptiste
11 years ago
There is nothing wrong with PHP namespaces, except that those 2 instructions give a false impression of package management.
... while they just correspond to the "with()" instruction of Javascript.

By contrast, a package is a namespace for its members, but it offers more (like deployment facilities), and a compiler knows exactly what classes are in a package, and where to find them.
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3
Anonymous
11 years ago
@ RS: Also, you can specify how your __autoload() function looks for the files. That way another users namespace classes cannot overwrite yours unless they replace your file specifically.
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-14
David Drakard
11 years ago
I agree with SR, the new namespaces feature has solved a number of problems for me which would have required horrible coding to solve otherwise.

An example use:
Say you are making a small script, and write a class to connect to a database, calling it 'connection'. If you find your script useful and gradually expand it into a large application, you may want to rename the class. Without namespaces, you have to change the name and every reference to it (say in inheriting objects), possibly creating a load of bugs. With namespaces you can drop the related classes into a namespace with one line of code, and less chance of errors.

This is by no means one of the biggest problems namespaces solve; I would suggest reading about their advantages before citicising them. They provide an elegant solutions to several problems involved in creating complex systems.
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-7
syed_992 at hotmail dot com
2 years ago
namespace declare must be starting of the script, only php tag can be written before namespace like this

<?php
namespace test;       //this is right way
class Testclass{/*****/}
function
test(/****/){}

?>
if you used any  other code like html tag or any javascript etc then it gives fatal error.

Example:
<html>
<head></head>
<body>
<?php
namespace test;       //Fatal Error
class Testclass{/*****/}
function
test(/****/){}

?>
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-20
Roadowl
5 years ago
quote:
Defining namespaces

(...)
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.

end quote.

So we have a title that talks 'defining' and a piece of text that talks 'declare' three times, one of which could be referring to some other 'declare' than the former two.

Please, documentation authors -- get your act together, finally.
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