There are 2 problems in this field.

1st: the description says: The current phase of workshop (0 = not available, 1 = submission, 2 = assessment, 3 = closed).

While the phase constants are
    const PHASE_SETUP                   = 10;
    const PHASE_SUBMISSION              = 20;
    const PHASE_ASSESSMENT              = 30;
    const PHASE_EVALUATION              = 40;
    const PHASE_CLOSED                  = 50;

2nd: The field is declared as optional but all workshops have the phase set to PHASE_SETUP on workshop_add_instance. If maintained as optional PHASE_SETUP should be the default param.