A strange question about mysql_close()

I want to ask a strange question. As we know, if we connect a mysql database in the same host twice with the same username, we won't get two new link, but two handle to only one link. Then, if I close one of them, whether the other one will be closed together? I wrote two pieces of code. The first is like this <?php $db1 = mysql_connect($host, $username, $pwd); $db2 = mysql_connect($host, $username, $pwd); $sql = 'show databases'; mysql_close($db1); $res = mysql_query($sql, $db2); $a = mysql_fetch_row($res); print_r($a); ?> Then the output of the $db2 is correct. So the $db2 isn't closed together with the $db1. But when I add a line among the code, the result is different <?php $db1 = mysql_connect($host, $username, $pwd); $db2 = mysql_connect($host, $username, $pwd); $sql = 'show databases'; mysql_close($db1); $db1 = true; //a new line $res = mysql_query($sql, $db2); $a = mysql_fetch_row($res); print_r($a); ?> Then the output is wrong. The PHP tell me that the link is not a valid one. I don't know how an assignment operation for the $db1 can close the $db2. Could someone help me. I'll appreciate your help.