When making two simultaneous connections to a MySQL server using the same username / password combination, the MySQL link resource returned is the same.  The upshot of this is that I am unable to use these 2 connections to simultaneously work with 2 different databases that happen to share one user/pass set.  

Put simply: If I call mysql_select_db() on one connection, it changes the selected DB on the other connection, too.  The following code illustrates the sharing of the connection by executing a bogus query on one connection then calling mysql_error() on both connections.

<?
	$conn1 = mysql_connect ("localhost", "sales", "xxxx");
	$conn2 = mysql_connect ("localhost", "sales", "xxxx");
	
	mysql_select_db("sales", $conn1);
	mysql_select_db("salesNEW", $conn2);
	
	mysql_query("SELECT blah FROM blah", $conn1);

	echo mysql_error($conn1)."<br />";
	echo mysql_error($conn2);
	
	echo "<br /><br />";

	echo $conn1."<br />";
	
	echo $conn2;

?>

The output is:

Table 'salesNEW.blah' doesn't exist
Table 'salesNEW.blah' doesn't exist

Resource id #2
Resource id #2


Notice that I'm executing the broken query on $conn1.  Yet the error message is returned on both $conn1 and $conn2.  Also notice that the Resource id's are both 2, which easily explains this behavior.

It gets more bizarre when I swap over to mysql_pconnect().  The output becomes:

Table 'salesNEW.blah' doesn't exist
Table 'salesNEW.blah' doesn't exist

Resource id #2
Resource id #3

So the behaviour is the same, but PHP clearly thinks it has 2 seperate connections this time.

To determine whether PHP or MySQL was causing this bug, I used 2 simultaneous instances of the mysql command line client and verified that it happily coped with the situation.