Would you suggest using a datetime or a timestamp field, and why (using MySQL)?

I'm working with PHP on the server side.

Timestamps in MySQL generally used to track change to records, and are often updated each time the record is changed. If you want to store a specific value you should use a datetime field.

If you meant that you want to decide linking using a UNIX timestamp or a native MySQL datetime field, go with the native format. You can do calculations within MySQL that way  ("SELECT DATE_ADD(my_datetime, INTERVAL 1 DAY)") and it is easy to change the arrange of the value to a UNIX timestamp ("SELECT UNIX_TIMESTAMP(my_datetime)") when you query the record if you want to operate on it with PHP.