关于命名空间需要注意的事项

No. Namespaces do not affect any existing code in any way, or any as-yet-to-be-written code that does not contain namespaces. You can write this code if you wish:

Example #1 Accessing global classes outside a namespace

<?php
$a = new \stdClass;

This is functionally equivalent to:

Example #2 Accessing global classes outside a namespace

<?php
$a = new stdClass;

How do I use internal or global classes in a namespace?

Example #3 Accessing internal classes in namespaces

<?php
namespace foo;
$a = new \stdClass;
function test(\ArrayObject $typehintexample = null) {}
$a = \DirectoryIterator::CURRENT_AS_FILEINFO;
// extending an internal or global class
class MyException extends \Exception {}
?>

How do I use namespaces classes, functions, or constants in their own namespace?

Example #4 Accessing internal classes, functions or constants in namespaces

<?php
namespace foo;
class MyClass {}
// using a class from the current namespace as a type hint
function test(MyClass $typehintexample = null) {}
// another way to use a class from the current namespace as a type hint
function test(\foo\MyClass $typehintexample = null) {}
// extending a class from the current namespace
class Extended extends MyClass {}
// accessing a global function
$a = \globalfunc();
// accessing a global constant
$b = \INI_ALL;
?>

How does a name like\my\nameor\nameresolve?

Names that begin with a\always resolve to what they look like, so\my\nameis in factmy\name, and\ExceptionisException.

Example #5 Fully Qualified names

<?php
namespace foo;
$a = new \my\name(); // instantiates "my\name" class
echo \strlen('hi'); // calls function "strlen"
$a = \INI_ALL; // $a is set to the value of constant "INI_ALL"
?>

How does a name likemy\nameresolve?

Names that contain a backslash but do not begin with a backslash likemy\namecan be resolved in 2 different ways.

If there is an import statement that aliases another name tomy, then the import alias is applied to themyinmy\name.

Otherwise, the current namespace name is prepended tomy\name.

Example #6 Qualified names

<?php
namespace foo;
use blah\blah as foo;
$a = new my\name(); // instantiates "foo\my\name" class
foo\bar::name(); // calls static method "name" in class "blah\blah\bar"
my\bar(); // calls function "foo\my\bar"
$a = my\BAR; // sets $a to the value of constant "foo\my\BAR"
?>

How does an unqualified class name likenameresolve?

Class names that do not contain a backslash likenamecan be resolved in 2 different ways.

If there is an import statement that aliases another name toname, then the import alias is applied.

Otherwise, the current namespace name is prepended toname.

Example #7 Unqualified class names

<?php
namespace foo;
use blah\blah as foo;
$a = new name(); // instantiates "foo\name" class
foo::name(); // calls static method "name" in class "blah\blah"
?>

How does an unqualified function name or unqualified constant name likenameresolve?

Function or constant names that do not contain a backslash likenamecan be resolved in 2 different ways.

First, the current namespace name is prepended toname.

Finally, if the constant or functionnamedoes not exist in the current namespace, a global constant or functionnameis used if it exists.

Example #8 Unqualified function or constant names

<?php
namespace foo;
use blah\blah as foo;
const FOO = 1;
function my() {}
function foo() {}
function sort(&$a)
{
    sort($a);
    $a = array_flip($a);
    return $a;
}
my(); // calls "foo\my"
$a = strlen('hi'); // calls global function "strlen" because "foo\strlen" does not exist
$arr = array(1,3,2);
$b = sort($arr); // calls function "foo\sort"
$c = foo(); // calls function "foo\foo" - import is not applied
$a = FOO; // sets $a to value of constant "foo\FOO" - import is not applied
$b = INI_ALL; // sets $b to value of global constant "INI_ALL"
?>

Import names cannot conflict with classes defined in the same file.

The following script combinations are legal:

file1.php

<?php
namespace my\stuff;
class MyClass {}
?>

another.php

<?php
namespace another;
class thing {}
?>

file2.php

<?php
namespace my\stuff;
include 'file1.php';
include 'another.php';
use another\thing as MyClass;
$a = new MyClass; // instantiates class "thing" from namespace another
?>

There is no name conflict, even though the classMyClassexists within themy\stuffnamespace, because the MyClass definition is in a separate file. However, the next example causes a fatal error on name conflict because MyClass is defined in the same file as the use statement.

<?php
namespace my\stuff;
use another\thing as MyClass;
class MyClass {} // fatal error: MyClass conflicts with import statement
$a = new MyClass;
?>

Nested namespaces are not allowed.

PHP does not allow nesting namespaces

<?php
namespace my\stuff {
    namespace nested {
        class foo {}
    }
}
?>

<?php
namespace my\stuff\nested {
    class foo {}
}
?>

Neither functions nor constants can be imported via theusestatement.

The only elements that are affected byusestatements are namespaces and class names. In order to shorten a long constant or function, import its containing namespace

<?php
namespace mine;
use ultra\long\ns\name;
$a = name\CONSTANT;
name\func();
?>

Dynamic namespace names (quoted identifiers) should escape backslash

It is very important to realize that because the backslash is used as an escape character within strings, it should always be doubled when used inside a string. Otherwise there is a risk of unintended consequences:

Example #9 Dangers of using namespaced names inside a double-quoted string

<?php
$a = new "dangerous\name"; // \n is a newline inside double quoted strings!
$obj = new $a;
$a = new 'not\at\all\dangerous'; // no problems here.
$obj = new $a;
?>

Undefined Constants referenced using any backslash die with fatal error

Any undefined constant that is unqualified likeFOOwill produce a notice explaining that PHP assumedFOOwas the value of the constant. Any constant, qualified or fully qualified, that contains a backslash will produce a fatal error if not found.

Example #10 Undefined constants

<?php
namespace bar;
$a = FOO; // produces notice - undefined constants "FOO" assumed "FOO";
$a = \FOO; // fatal error, undefined namespace constant FOO
$a = Bar\FOO; // fatal error, undefined namespace constant bar\Bar\FOO
$a = \Bar\FOO; // fatal error, undefined namespace constant Bar\FOO
?>

Cannot override special constants NULL, TRUE, FALSE, ZEND_THREAD_SAFE or ZEND_DEBUG_BUILD

Any attempt to define a namespaced constant that is a special, built-in constant results in a fatal error

Example #11 Undefined constants

<?php
namespace bar;
const NULL = 0; // fatal error;
const true = 'stupid'; // also fatal error;
// etc.
?>

There is a way to define a namespaced constant that is a special, built-in constant, using define function and setting the third parameter case_insensitive to false:
<?php
namespace foo;
define(__NAMESPACE__ . '\NULL', 10); // defines the constant NULL in the current namespace
var_dump(NULL); // will show 10
var_dump(null); // will show NULL
?>
 No need to specify the namespace in your call to define(), like it happens usually
<?php
namespace foo;
define(INI_ALL, 'bar'); // produces notice - Constant INI_ALL already defined. But:
define(__NAMESPACE__ . '\INI_ALL', 'bar'); // defines the constant INI_ALL in the current namespace
var_dump(INI_ALL); // will show string(3)"bar". Nothing unespected so far. But:
define('NULL', 10); // defines the constant NULL in the current namespace...
var_dump(NULL); // will show 10
var_dump(null); // will show NULL
?>
 If the parameter case_insensitive is set to true
<?php
namespace foo;
define (__NAMESPACE__ . '\NULL', 10, true); // produces notice - Constant null already defined
?>

When creating classes or calling static methods from within namespaces using variables, you need to keep in mind that they require the full namespace in order for the appropriate class to be used; you CANNOT use an alias or short name, even if it is called within the same namespace. Neglecting to take this into account can cause your code to use the wrong class, throw a fatal missing class exception, or throw errors or warnings.
In these cases, you can use the magic constant __NAMESPACE__, or specify the full namespace and class name directly. The function class_exists also requires the full namespace and class name, and can be used to ensure that a fatal error won't be thrown due to missing classes.
<?php
namespace Foo;
class Bar {
  public static function test() {
    return get_called_class();
  }
}
namespace Foo\Foo;
class Bar extends \Foo\Bar {
}
var_dump( Bar::test() ); // string(11) "Foo\Foo\Bar"
$bar = 'Foo\Bar';
var_dump( $bar::test() ); // string(7) "Foo\Bar"
$bar = __NAMESPACE__ . '\Bar';
var_dump( $bar::test() ); // string(11) "Foo\Foo\Bar"
$bar = 'Bar';
var_dump( $bar::test() ); // FATAL ERROR: Class 'Bar' not found or Incorrect class \Bar used

[Editor's note: that behavior is caused by a bug in PHP 7.0, which has been fixed as of PHP 7.0.7.]
Regarding the entry "Import names cannot conflict with classes defined in the same file".
- I found that since PHP 7.0 this is no longer the case.
In PHP 7.0 you can have a class with a name that matches an imported class (or namespace or both at the same time).
<?php
namespace ns1 {
 class ns1 {
  public static function write() {
   echo "ns1\\ns1::write()\n";
  }
 }
}
namespace ns1\ns1 {
 class ns1c {
  public static function write() {
   echo "ns1\\ns1\\ns1c::write()\n";
  }
 }
}
namespace ns2 {
 use ns1\ns1 as ns1; // both a class in ns1, and a namespace ns1\ns1
  
 // the next class causes fatal error in php 5.6, not in 7.0
 class ns1 {
  public static function write() {
   echo "ns2\\ns1::write()\n";
  }
 }
  
 ns1::write(); // calls imported ns1\ns1::write()
 ns1\ns1c::write(); // calls imported ns1\ns1\ns1c::write()
 namespace\ns1::write(); // calls ns2\ns1::write()
}
?>

Regarding "Neither functions nor constants can be imported via the use statement." Actually you can do it in PHP 5.6+:
<?php
// importing a function (PHP 5.6+)
use function My\Full\functionName;
// aliasing a function (PHP 5.6+)
use function My\Full\functionName as func;
// importing a constant (PHP 5.6+)
use const My\Full\CONSTANT;
?>

To correct manolachef's answer: define() ALWAYS defines constants in the GLOBAL namespace.
As nl-x at bita dot nl states in the note at http://www.php.net/manual/en/function.define.php, the constant "NULL" can be defined with define() case-sensitively, but can only be retrieved with constant(), leaving the meaning of NULL uppercase keyword as the only value of the type null.