[RESOLVED] What am I missing?

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What am I missing here? This works just fine somewhere else. I have made only very minor changes.

I get error: "mysql_num_rows(): supplied argument is not a valid MySQL result resource"

$sql = "select first_name, last_name, username, password, from sites where username='$username' and password='$password'";

$result = mysql_query($sql);
$num = mysql_num_rows($result);

Thank you.

[m_tt]

The query is failing, try enabling error_reporting. Or...

$result = mysql_query($sql)or die(mysql_error());

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Thanks m@tt.