supplied argument is not a valid MySQL result resource

Greetings,

I have a problem which shows up *intermittently* when executing a simple query. Basically, mysql_query() returns no resource *some* of the time:

"PHP Warning: mysql_num_rows(): supplied argument is not a
valid MySQL result resource in /home/www/..."

When I trap the error using mysql_error(), I get the following:
"No Database Selected"

For kicks, I altered the query to use the fully qualified table name (<db>.<table>), and the error changed to:
"Access denied for user: 'username@%' to database 'db'"

I was able to replicate this on a browser as well as using "ab" with no concurrency. The system will sometimes handle 1000 or more queries without any problems, and then a bunch of errors will appear.

I cannot roll this out into a production environment until this is fixed. I would really appreciate any advice.

Here's the relevant code:

$conn = mysql_connect(CHAR_DB,CHAR_DB_USER,CHAR_DB_PASS) or die("Cannot connect to db.");

mysql_select_db(USER_DB, $conn);

$sql = "SELECT cash FROM cash WHERE id = '$id'";

$res = mysql_query($sql);

if (!$res) {
error_log('Invalid query: ' . mysql_error());
}

$numrows = mysql_num_rows($res);

Thanks!

A simple debugging technique...

Add this to your code:
$res = mysql_query($sql) or die("Error ".mysql_errno().": ".mysql_error()."<br>\nThe SQL sent was: $sql");



Refer to the PHP documentation if you are unsure of what these functions do:

• die()
• mysql_errno()
• mysql_error()