XHP: Setup

Namespace Support

Support for namespaced XHP classes (elements) is enabled by default since HHVM 4.73. It comes with many related XHP changes compared to previous versions.

We recommend using this version or newer, since it's more thoroughly tested and doesn't require any extra configuration, but XHP namespace support can also be enabled in older HHVM versions (since around HHVM 4.46) by adding the following flags to your .hhconfig:

enable_xhp_class_modifier = true
disable_xhp_element_mangling = true

And to hhvm.ini (or provided via -d when executing hhvm):


In HHVM 4.73 or newer, XHP namespace support can be disabled by setting these to false.

If these flags are disabled, or using an older version of HHVM:

  • XHP classes cannot be declared in namespaces (only in the global namespace)
  • any code that uses XHP classes also cannot be in a namespace, as HHVM previously didn't have any syntax to reference XHP classes across namespaces
  • note that the above two rules are not consistently enforced by the typechecker or the runtime, but violating them can result in various bugs
  • it is, however, possible to use namespaced code from inside XHP class declarations

Make sure to also use the correct version of XHP-Lib (see below) based on whether XHP namespace support is enabled in your HHVM version.

Additional Configuration Flags

These are not enabled by default in any HHVM version, but we recommend enabling them in any new Hack projects:

  • disable_xhp_children_declarations = true disables the old way of declaring allowed children, which has been deprecated in favor of the Facebook\XHP\ChildValidation\Validation trait. See Children for more information.
  • check_xhp_attribute = true enables the typechecker to check that all required attributes are provided. Otherwise, these would only be errors at runtime.

The XHP-Lib Library

While the XHP syntax is part of Hack, a large part of the implementation is in a library called XHP-Lib that needs to be installed via Composer (add facebook/xhp-lib to your composer.json manually or by running composer require facebook/xhp-lib ^4.0 or ^3.0).

This includes the base classes and interfaces, and definitions of standard HTML elements.

XHP-Lib Version

There are currently two major supported versions of XHP-Lib:

  • XHP-Lib v4: to be used when XHP namespace support is enabled. Declares all base classes, interfaces and elements in namespaces (e.g. standard HTML elements are in Facebook\XHP\HTML). It is also more strict (e.g. disallows most mutations after an element is rendered) and deprecates some features (e.g. attribute "clobbering" in XHPHTMLHelpers).
  • XHP-Lib v3: to be used in older HHVM versions or when XHP namespace support is disabled. Declares everything in the global namespace, with the exception of Facebook\XHP\ChildValidation.

All the following guides are written with the assumption that XHP namespace support is enabled and XHP-Lib v4 is used, but there are notes pointing out any major differences—look for Historical note sections.

XHP namespaces are not enabled in Facebook's WWW repository, so all Historical note sections apply.