Get the current signed request being used by the SDK. A signed request is a concatenation of a HMAC SHA-256 signature string, a period (.), and a base64url encoded JSON object. It looks something like this (without the newlines):
vlXgu64BQGFSQrY0ZcJBZASMvYvTHu9GQ0YM9rjPSso . eyJhbGdvcml0aG0iOiJITUFDLVNIQTI1NiIsIjAiOiJwYXlsb2FkIn0
See the Signed Request documentation page for more information.
$signed_request = $facebook->getSignedRequest();