我在UPDATE时可以从mySQL数据库中检索变量吗？

I'm running this query:

if($ModifiedInGoogle != $row['ModifiedInGoogle']) {
  // run update query
  mysql_query("UPDATE events 
                  SET EventName = '$EventName', 
                      StartDate = '$StartDate', 
                WHERE GoogleID = '$GoogleID' ");

If the EventID variable is not being changed in the UPDATE, is there a way I can get it and assign it to an $EventID variable here?

No. UPDATE queries are for putting data into the database. If you want to retrieve data, then you use a separate SELECT query.