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Namespaces

A namespace is used to prevent the name collisions of constants, functions, classes and interfaces. Its declaration must precede all other statements in a file, including any HTML code. The only statement that can precede it is the declare() statement used for the encoding directive.
A namespace can take on the brace {} form, or the free statement form. However, the two different forms must not appear within a single file.
RESETRUNFULL
bar.php:
<?php  //bar.php, namespaces in {} form
namespace Bar {
   const a=10;  // defined as \Bar\a
}
namespace {
   const a=50;  // defined as \a, in the global namespace
}
?>

<?php  //foo.php, namespaces in free statement form
namespace Foo;
include "bar.php";
const a=20;           // defined as \Foo\a;
echo a;                  // 20, resolves to \Foo\a
echo \Foo\a;          // 20, resolves to \Foo\a
echo \Bar\a;          // 10, resolves to \Bar\a
echo \a;                // 50, resolves to \a, the global namespace
echo namespace\a; // 20, resolves to \Foo\a
//echo Foo\a;         // error, resolves to \Foo\Foo\a
//echo Bar\a;         // error, resolves to \Foo\Bar\a

namespace Foo\Alpha;
const a=30;           // defined as \Foo\Alpha\a
echo a;                  // 30, resolves to \Foo\Alpha\a;

namespace Foo;
echo Alpha\a;      // 30, resolves to \Foo\Alpha\a;
echo strlen("hi")  // 2, fall back to \strlen(), the global nspc;

?>

<!DOCTYPE html>
<html>
<head></head>
<body><?php
include "foo.php";
echo a;         // 50, resolves to \a
echo \Bar\a;  // 10, resolves to \Bar\a
?>
</body>
</html>
If a namespaced function or constant does not exist, PHP will automatically fall back to global functions or constants. On the other hand, class names always resolve to the current namespace.

We can use an alias for a class name, an interface name or a namespace.
A namespace can take on the brace {} form, or the free statement form. However, the two different forms must not appear within a single file.
RESETRUNFULL
alpha_beta.php:
<?php  //alpha_beta.php
namespace Alpha\Beta;
class MyClass{
   public static $v=100;
}
?>

foo-2.php:
<?php  //foo2.php
namespace Foo;
include "alpha_beta.php";

$o=new \Alpha\Beta\MyClass;
$ns='Alpha\Beta\MyClass'; // no leading \ needed
$o2=new $ns;
echo $o2::$v;

use Alpha\Beta as A;   // aliasing a namespace
echo A\MyClass::$v;

use Alpha\Beta\MyClass as C;  // aliasing a class name
echo C::$v;

use Alpha\Beta\MyClass;  // Alpha\Beta\MyClass as MyClass;
echo MyClass::$v;

use Alpha\Beta\MyClass as D, Alpha\Beta;  // Multiple decl.
echo Beta\MyClass::$v;
?>

<!DOCTYPE html>
<html>
<head></head>
<body><?php
include "foo-2.php";
?>
</body>
</html>