is this how to update a field in a migration that references other fields?

migrate up pfsh cigarette use

	public function up()
	{
		$patientEvals = PatientEvaluation::all();
		foreach ($patientEvals as $eval) {
			$eval->pfsh['social_history']['tobacco_use'] = 
				$eval->pfsh['social_history']['cigarette_use'] + ' - ' + 
				$eval->pfsh['social_history']['tobacco_use'];

			$eval->save();
		}
	}