How to refer to a custom field of the type menu in PHP

Summary

How to refer to a custom field of the type menu in PHP

Content

Hi there,

I am having a problem to update a CUSTOM FIELD of the type MENU
Can anyone tell me how to refer to this field correctly?
I've tried the following 3 syntaxes with no success:

$gender=$_POST["Incident_CustomFields_PatientInfo_Patient_Gender"];

$incident->CustomFields->PatientInfo->Patient_Gender = $gender; 
ERROR 500

-OR-

$incident->CustomFields->PatientInfo->Patient_Gender_Menu = $gender; 
​ERROR 500

-OR-

 $incident->CustomFields->PatientInfo->Patient_Gender->Patient_Gender_Menu = $gender;
No error - response 200 Ok but not is written to the database

$incident->save(RNCPHP\RNObject::SuppressAll);