Просторни имиња

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.

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!');
?>

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.
?>

"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 {
}
?>

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
?>

namespace statement  is defined at first of the php files. But 
    before namespace declaration only three elements allowed.
      1.declare statement
      2.spaces
      3.comments

@ 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.

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.

Namespace name are case-insensitive.
namespace App
and
namespace app
are same meaning.

Besides, Namespace keword are case-insensitive.
Namespace App
namespace App
and
NAMESPACE App
are same meaning.

Please note that a PHP Namespace declaration cannot start with a number.
It took some time for me to debug...