title: PHP about single instance connect mysql
body: <p>In many php code of online, they use php single instance to connect mysql database.  of course, they use pdo connection in the single instance.  but i have a question about the database connection timeout. 
      as we known, the single instance will destroy as we use it out, the connection will cut by the time . if we use the instance ten time, the connection will be establish and destroy ten times. that is not the result i want.  Is there any methods to deal with the problem.  I don't figure out that if it is wise to save the instance for some time. for example, save it in the memory.  </p>