Add subject-id signal constant

Author: tvdijen

src/SAML2/Constants.php

@@ -36,6 +36,11 @@ class Constants
      */
     const ATTR_PAIRWISE_ID = 'urn:oasis:names:tc:SAML:attribute:pairwise-id';
 
+    /**
+     * Subject signal attribute
+     */
+    const PROFILE_SUBJECT_ID_REQ = 'urn:oasis:names:tc:SAML:profiles:subject-id:req';
+
     /**
      * The URN for the HTTP-POST binding.
      */