Sessions
PHP Manual

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().

Sessions
PHP Manual