select data from database with values from multiple checkbox array

this is the code i m getting error with:-

function sanitizeString($var)
    {
        $var = trim($var);
        $var = stripslashes($var);
        $var = strip_tags($var);
        $var = htmlentities($var);
        return $var;
    }
    $errors = array();
    $query = "";
    if (isset($_POST["tests_list"])) {
        $test_id = mysqli_real_escape_string($con, sanitizeString($_POST["tests_list"]));
        $can_ids = $_POST["checkCandidate"];
        $status = "active";
        foreach ($can_ids as $can_value) {
            $sel = mysqli_query($con, 'select * from assign_table where candidate_id="' .$can_value.'" and test_id="$test_id"');
            if(mysqli_num_rows($sel)>0){
                $errors[] = $can_value  . " already assigned<br>";
            }else{
                $query .= "insert into assign_table(test_id, candidate_id, status) values('$test_id', '" . $can_value . "', '$status');";
            }
        }
        if(count($errors)===0){
            if (mysqli_multi_query($con, $query)) {
                echo "success";
            } else {
                echo "error";
            }
        }else{
            foreach ($errors as $error) {
                echo $error;
            }
        }
    } else {
        echo "Direct Access Not Allowed";
    }

i want to check if selected candidate already assigned or not. if candidate is already assigned to test it should display message already assigned, else insert candidate to assign_table, nut its not validating the select query. Anyone can help?