Resource Types
Examples
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().

Resource Types
Examples
Sessions
PHP Manual