Source Code Fundamentals: Namespaces

A namespace is a container for a set of (typically related) definitions of classes, interfaces, traits, functions, and constants.

When the same namespace is declared across multiple scripts, and those scripts are combined into the same program, the resulting namespace is the union of all of the namespaces' individual components.

The Root Namespace

In the absence of any namespace definition, the names of subsequent classes, interfaces, traits, functions, and constants are in the root namespace, which is not named.

Some types, such as Exception, and constants, and library functions (such as sqrt) are inherited from PHP and, as such, are also defined outside a namespace. Prefix a backslash (\) to refer to these types or functions; for example: \Exception, \sqrt.


A namespace can have sub-namespaces, where a sub-namespace name shares a common prefix with another namespace.

For example, the namespace Graphics can have sub-namespaces Graphics\TwoD and Graphics\ThreeD, for two- and three-dimensional facilities, respectively.

Apart from their common prefix, a namespace and its sub-namespaces have no special relationship. For example, NS1\Sub can exist without NS1. A named top-level namespace does not need to exist for a sub-namespace to exist.

Reserved Namespaces

The namespaces HH, PHP, php, and sub-namespaces beginning with those prefixes are reserved for use by Hack.

XHP and the HTML Namespace

As of HHVM 4.73 and XHP-Lib v4, standard XHP elements like <p> are defined in Facebook\XHP\HTML (for this example, specifically Facebook\XHP\HTML\p).

For more information, see XHP Namespace Syntax.

Special Constants

When debugging, use the predefined constant __NAMESPACE__ to access the name of the current namespace.

Declaring a Namespace

Namespace declarations can be file-scoped with namespace MyNS;, or block-scoped with namespace MyNS { ... }.

With semicolons, a namespace extends until the end of the script, or until the next namespace declaration, whichever is first.

namespace NS1;
...				// __NAMESPACE__ is "NS1"
namespace NS3\Sub1;
...				// __NAMESPACE__ is "NS3\Sub1"

With brace delimiters, a namespace extends from the opening brace to the closing brace.

namespace NS1
...                   // __NAMESPACE__ is "NS1"
...                   // __NAMESPACE__ is ""
namespace NS3\Sub1;
...                   // __NAMESPACE__ is "NS3\Sub1"

Importing from other Namespaces

With the use keyword, a namespace can import one or more member names into a scope, optionally giving them each an alias.

When importing many names, use { ... }.

use namespace NS1\{C, I, T}; // instead of `NS1\C, NS1\I, NS1\T`

Imported names can designate a namespace, a sub-namespace, a class or interface or trait, a function, or any built-in type.

If an imported name introduces ambiguity, you can refer to name foo with namespace\foo—using the the actual word namespace. For example, if you're importing a function bar(), but also want to call the bar() function from within your own namespace, refer to the one native to your namespace with namespace\bar().

namespace NS1 {
  const int CON1 = 100;
  function f(): void {
    echo "In ".__FUNCTION__."\n";

  class C {
    const int C_CON = 200;
    public function f(): void {
      echo "In ".__NAMESPACE__."...".__METHOD__."\n";

  interface I {
    const int I_CON = 300;

  trait T {
    public function f(): void {
      echo "In ".__TRAIT__."...".__NAMESPACE__."...".__METHOD__."\n";

namespace NS2 {
  use type NS1\{C, I, T};

  class D extends C implements I {
    use T;

  function f(): void {
    $d = new D();
    echo "CON1 = ".\NS1\CON1."\n";
namespace Hack\UserDocumentation\Fundamentals\Namespaces\Examples\Main;

function main(): void {
CON1 = 100
In NS1\f
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