PHP - mysql_insert_id() and race conditions?

Question

I think I've got a race condition within a bit of PHP code whereby mysql_insert_id() is returning 0 - but I can't work out why.

$insertsess=mysql_query("insert into cior_sessions values ('','$cid','$uid','$tutook',STR_TO_DATE('$sessiondate','%d/%m/%Y'),'$sessionnum')");
$sessionid=mysql_insert_id($insertsess);
echo "<input type=\"hidden\" name=\"sid\" id=\"sid\" value=\"" . $sessionid . "\">";

I've not been able to replicate the problem myself, but can see that other users have (by virtue of the fact that what they're subsequently doing is resulting of an SID of 0, but not consistently).

My understanding of this was that mysql_insert_id would wait for the query from $insertsess to complete - is this wrong? If so, do I need to have something such as a watchdog loop which waits for mysql_insert_id to return something other than zero? That seems a bit stupid. :/

Answer 1

When you call mysql_insert_id() you should be using the connection resource, not the result set resource as you are doing in the snippet you posted.

$link = mysql_connect('dbhost', 'dbuser', 'dbpass', 'dbname');
$result = mysql_query("Some query here');
$lastId = mysql_insert_id($link); // you have used your result set here