Is there a reason you need to import customers via PHP?
You can import customers as a CSV right within SambaPOS using Batch Create Entities. I can see you were given details of this before.
I have added customers using the Batch Create Entities many times without problem. The issue you got there is most likely due to the data you tried to import, but I note you did not share an example.
I recommend you use the Batch Create Entities feature and do not pursue using this PHP method as it is is unnecessary.