Description:
------------
Preface: I *do* understand that by default MySQL connections are shared in PHP.
 I also note that in older versions a single mysql_close() would close all of the links; see #9107: there now there appears to be reference counting before finally closing the TCP connection -- which is IMHO better than the older behaviour, but the implementation has its own bugs:

 PHP appears to keeps an internal count of the number of times a link has been duplicated. When the link count is <= 0, the underlying TCP connection is actually closed.
 * close() reduces the link count by 1
 * setting the connection to null *also* reduces the link count by 1 -- even if that link has already been close()d

 Currently the only workarounds for this are to:

[1] Set new_link to true every time you mysql_connect() -- potentially creating a lot of TCP connections and slowing the program down

[2] close() the link, but never set it to null and hope that PHP won't clean it up until the end of the program: This *will* fail sometimes though; see the example at http://www.levi.id.au/mysql4.php.txt

[3] Never mysql_close() links, only set them to null and hope that PHP will in fact clean up the TCP connection before MySQL runs out of available connections (admittedly only a problem when you have a lot of simultaneous connections to your database) -- this does work now, but we're not supposed to assume anything about when PHP does its object destruction.

 The third is really the only viable solution; but is dependent on the internal implementation of the MySQL extension.



 At best, the current situation is that if you ever have shared links, you should never call mysql_close if you ever expect to use that database again in your program.

Reproduce code:
---------------
Simple example:

#!/usr/local/bin/php -q
<?
$conn1 = mysql_connect('localhost:3306', 'levi', 'DaCr0n!');
$conn2 = mysql_connect('localhost:3306', 'levi', 'DaCr0n!');

mysql_select_db('surveytest', $conn1);
mysql_select_db('surveytest', $conn2);

mysql_close($conn1); $conn1 = null;
mysql_close($conn2); $conn2 = null;

?>

See also the example at http://www.levi.id.au/mysql4.php.txt

Expected result:
----------------

Blank output.


Actual result:
--------------
PHP Warning:  mysql_close(): 1 is not a valid MySQL-Link resource in /home/levi/public_html/mysql2.php on line 10
<br />
<b>Warning</b>:  mysql_close(): 1 is not a valid MySQL-Link resource in <b>/home/levi/public_html/mysql2.php</b> on line <b>10</b><br />



(If I remove the mysql_close($conn1); it works)
(If I remove the $conn1 = null; it also works)