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Just to clarify... A connection is to a server, not a table. The database name, when connecting, is just the default name to use if no db name is specified in the query. In this case, both connections are to "localhost" so a single connection will suffice. If your tables were on different servers then you would need 2 connections and you would have to query the tables in separate queries,

One does not need to make 2 connections. Simply add the dbname to the tablename in your one query. "select a.fld1, b.fld1,a.host, a.child, a...., b..., b... from db1.tablea a, db2.tableb b where a.key = b.key

first of all you should use an unique index for email and I don't understand the also having for username (though that too). Though I now can see both...tired. Second take a look at this $sql = "SELECT * FROM register WHERE username:username AND email:email"; See anything missing? I give you a hint it's between username :username and also email :email. Here's a good link https://phpdelusions.net/pdo and I even still use it from time to time.