php - mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, boolean given

I am pretty new to PHP and MySQL and I just can't figure this one out. I have searched all around the forum but haven't found an answer I can make sense of. I originally was using mysql_fetch_assoc() but I could only search numbers and I received errors when searching for letters as well. I hope I am on the right track here. Thank you in advance for all your help!

$con = mysqli_connect($hostname,$username,$password) or die ("");
mysqli_select_db($con, $dbname);

if (isset($_GET['part'])){
    $partid = $_GET['part'];
    $sql = 'SELECT * 

        FROM $usertable 
        WHERE PartNumber = $partid';

    $result = mysqli_query($con, $sql);
    $row = mysqli_fetch_assoc($result);

    $partnumber = $partid;
    $nsn = $row["NSN"];
    $description = $row["Description"];
    $quantity = $row["Quantity"];

    $condition = $row["Conditio"];
}

Answer

This happens when your result is not a result (but a "false" instead).
You should change this line

$sql = 'SELECT * FROM $usertable WHERE PartNumber = $partid';

to this:

$sql = "SELECT * FROM $usertable WHERE PartNumber = $partid";

because the " can interprete $variables while ' cannot.

Works fine with integers (numbers), for strings you need to put the $variable in single quotes, like

$sql = "SELECT * FROM $usertable WHERE PartNumber = '$partid' ";

If you want / have to work with single quotes, then php CAN NOT interprete the variables, you will have to do it like this:

 $sql = 'SELECT * FROM '.$usertable.' WHERE string_column = "'.$string.'" AND integer_column = '.$number.';