You can perform a SQL query against more than one database with PHP. Here is a sample generic SQL query against two databases:

SELECT image.image_title, greetings.date
FROM db1.image, db2.greetings
WHERE greetings.image_id=image.imageid

And here is some PHP sample code on how you would query two databases in a single SQL statement:

$query="INSERT INTO db1.table1 SELECT * FROM db2.table2 WHERE a=1";
mysql_query($query,$connection);