为什么这个sql语句一直说它是布尔值而不是参数？ （PHP / MySQL的）

Question

In this statement, I am trying to see if there is the latest posting in the database that has exactly same title, price, city, state, detail. If there is, then it would say to the user that the exact post has been already made; if not then insert the posting into the dbc. (This is one type of check so that users can't accidentally post twice. This may not be the best check, but this statement error is annoying me, so I want it to work :))

Why won't this sql work? I think it's not letting the title=$title and not getting the value in the $title...

> ERROR: mysqli_num_rows() expects
> parameter 1 to be mysqli_result,
> boolean given in postad.php on line
> 365

    //there is a form that users fill out that has title, price, city, etc
    <form>
    blah blah
    </form>

         //if users click submit, then does all the checks and if all okay, insert to dbc
        if (isset($_POST['submit'])) 
        {

        // Grab the pposting data from the POST and gets rid of any funny stuff
        $title = mysqli_real_escape_string($dbc, trim($_POST['title']));
        $price = mysqli_real_escape_string($dbc, trim($_POST['price']));
        $city = mysqli_real_escape_string($dbc, trim($_POST['city']));
        $state = mysqli_real_escape_string($dbc, trim($_POST['state']));
        $detail = mysqli_real_escape_string($dbc, trim($_POST['detail']));

         if (!is_numeric($price) && !empty($price))
         {
            echo "<p class='error'>The price can only be numbers. 
                No special characters, etc</p>";
        }

        //Error problem...won't let me set title=$title, detail=$detail, etc.            
         //this statement after all the checks so that none of the variables are empty
        $query="Select * FROM posting 
    WHERE user_id={$_SESSION['user_id']}
    AND title=$title
    AND price=$price
    AND city=$city
    AND state=$state
    AND detail=$detail";
    $data = mysqli_query($dbc, $query);
    if(mysqli_num_rows($data)==1) 
    {
        echo "You already posted this ad. Most likely caused by refreshing too many times.";
    }

}

Answer 1

Try num rows condition like this

if(mysqli_num_rows($data)>0) {
}