How to run database class methods inside other classes?

I am new to using classes in PHP, I just realized, in most of my class methods, I need to do some mysql queries. I have another class that handles my connection and queries but how should I go about calling that class from other classes? Doesn't this k开发者_如何学运维ind of defeat the purpose of using classes if all my classes rely on my DB class?

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this is usually solved with the Singleton or Factory pattern...

when you need to use the db, you snatch the app's db object:

$db = Site::getDb();   // singleton
$db->exec('update t set i = i + 1');

getDb returns a static instance.

or:

 $db = Db::getDb('dsn');   // factory
 $db->exec('update t set i = i + 1');

returns a static instance if it exists, or creates a new db handle for that dsn...