Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given in C:\x

please help me i am new to PHP and i am having this error on my program
(Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given in C:\xampp\htdocs\project\viewforum.php on line 29)
<?php
include("config.php");
if(isset($_GET['id']) == TRUE) {
if(is_numeric($_GET['id']) == FALSE) {
header("Location: " . $config_basedir);
}
$validforum = $_GET['id'];
}
else {
header("Location: " . $config_basedir);
}
require("header.php");
$forumsql = "SELECT * FROM forums…
Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given in C:\x

See original post by exzevia