I could be wrong here, but that sounds like two different things. The cookie is an element of the client's browser which is only really used when the user identifies themself on a HubSpot form. Then their data is logged within HubSpot to their contact record. https://knowledge.hubspot.com/account/how-does-hubspot-track-visitors
The PHP SDK is used so that your external server(s) can communicate to HubSpot.
Hope that helps and please correct me if I am totally missing something.
I could be wrong here, but that sounds like two different things. The cookie is an element of the client's browser which is only really used when the user identifies themself on a HubSpot form. Then their data is logged within HubSpot to their contact record. https://knowledge.hubspot.com/account/how-does-hubspot-track-visitors
The PHP SDK is used so that your external server(s) can communicate to HubSpot.
Hope that helps and please correct me if I am totally missing something.