Locked Session Handler: Session handler with variable locking support

This class is custom session handler that allows for values stored in the session variables to be locked to prevent changes.

The class provides functions to set, retrieve, check and delete session variable values, as well functions to lock and unlock given session variables.

The names of locked variables are added to a special session variable array. If a script attempts to changes or delete locked session variables, they are not changed and the class will issue a non-fatal error notice.