Warning: mysql_num_rows(): supplied argument is not a valid

Hi i'm receiving the error below, i've double checked everything and don't know why its being displayed. Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /home/namebob/public_html/site_reg/statlookup.php on line 6

<?php
    include_once('common.php');
    $cid        =   $_REQUEST['cid'];
    $_elmID  开发者_C百科   =   $_REQUEST['elmID'];
    $scan = mysql_query("SELECT  `id`, `state` FROM `mast_state` WHERE `countryid` = $cid");
    if(mysql_num_rows($scan)>0)
    {
...

$scan = mysql_query("SELECT  `id`, `state` FROM `mast_state` WHERE `countryid` = $cid");
    $stock_num = '';
    if($scan )
    {
        while($row=mysql_fetch_object($scan))
        {
            if($row->state)
            $state  = $row->state;
        }   
    }       

It's worth trying var_dump($scan), if you get something like null / false, that means the query fails. If that's the case, try echo mysql_error() to check for any sql error (missing table etc)

$scan = mysql_query("SELECT  `id`, `state` FROM `mast_state` WHERE `countryid` = '".mysql_real_escape_string($cid)."'");

But really, use PDO.

$stm = $db->prepare("SELECT  `id`, `state` FROM `mast_state` WHERE `countryid` = ?");
if( $stm && $stm->execute(array($cid)) ) {
  while( $data = $stm->fetch(PDO::FETCH_ASSOC) ) {

  }
}

Most probably your query is failing. Try:

if($scan) {
  if(mysql_num_rows($scan) > 0) {
    //...
  }
} else {
   trigger_error(mysql_error()); 
};