How to write a MySQL query on 2 different tables located in two different databases on two different servers?

I think you can use 2 different connections:

$connection1= mysql_connect('server1', 'user1', 'password1');
$connection2= mysql_connect('server2', 'user2', 'password2');

Extract data from database 1 and use it to query database 2. Or vice versa, depending on what you need. This means that you will use 2 queries.

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