Multiple error messages upon upgrade to CiviCRM 4.2.20
Those are warnings, not errors. It's simply PHP telling you that CiviCRM 4.2 is a rather old piece of software (which it is!). They will not prevent CiviCRM from working normally, certainly not for the brief time between now and when you get it upgraded further.

I recommend you ignore them and proceed with the upgrade. The latest version of CiviCRM is more up-to-date so will not trigger php warnings.