Problem:mysql_fetch_assoc(): supplied argument is not a valid MySQL result resource

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You should be error checking every call to mysql which could fail. mysql_select_db() is the same as calling mysql_query('use dbname').

if (! mysql_select_db($db)) {
    echo(mysql_error());
    die;
}

if (! ($result = mysql_query($query)) {
    echo(mysql_error());
    die;
}