PHP Login Page Returns Error 500

I have written a page that allows users to sign in. But it returns error 500. It will at least pass a specified error message if I comment out the $resultCheck variable. What is wrong with my $resultCheck? I'm using $conn for the signup page so I know that works. I also know $sql is correct because I ran the same command through MariaDB and it worked. I have a signup form that works properly so there is no issue with PHP interacting with MariaDB. I also have 0 syntax errors. Why on earth would it be giving me an error 500?

<?php

session_start();

if (isset($_POST['submit']))
{

include 'serv.db.php';

$uid = mysqli_real_escape_string($conn, $_POST['uid']);
$pwd = mysqli_real_escape_string($conn, $_POST['pwd']);

//check if inputs are empty
if (empty($uid) || empty($pwd))
    {
    header("Location: signin.php?login=empty");
    exit();
    }   
else
    {

    $sql = "SELECT * FROM users WHERE user_uid='$uid';";
    $result = mysqli_query($conn, $sql);
    $resultCheck = mysqli_num_rows($result);
    if ($resultCheck < 1)
        {
            header("Location: signin.php?login=error");
            exit();     
        }
    else
        {
            if ($row = mysql_fetch_assoc($result))
            {
                //echo $row['user_uid'];
                //de-hash password
                $hashedPwdCheck = password_verify($pwd, $row['user_pwd']);
                if ($hashedPwdCheck == false)
                {
                    header("Location: signin.php?login=error");
                    exit();
                }
                elseif ($hashedPwdCheck == true)
                {
                    //Sucessfully create user here.
                    $_SESSION['u_id'] = $row['user_id'];
                    $_SESSION['u_first'] = $row['user_first'];
                    $_SESSION['u_last'] = $row['user_last'];
                    $_SESSION['u_email'] = $row['user_email'];
                    $_SESSION['u_uid'] = $row['user_iid'];
                    header("Location: signin.php?login=success");
                    exit();
                }
            }
        }
    }
}
else
{
header("Location: signin.php?");
exit();
}