# Predefined Constants

The constants below are defined by this extension, and will only be available when the extension has either been compiled into PHP or dynamically loaded at runtime.

`SID` (string)
  Constant containing either the session name and session ID in the form of `"name=ID"` or empty string if session ID was set in an appropriate session cookie. This is the same id as the one returned by session_id().
`PHP_SESSION_DISABLED` (int)
  Since PHP 5.4.0. Return value of session_status() if sessions are disabled.
`PHP_SESSION_NONE` (int)
  Since PHP 5.4.0. Return value of session_status() if sessions are enabled, but no session exists.
`PHP_SESSION_ACTIVE` (int)
  Since PHP 5.4.0. Return value of session_status() if sessions are enabled, and a session exists.