PHP Login script problems

i was wondering if anyone could help as i am trying to adapt a PHP login script to accept only the password without the need for the username and I don't know what i am doing wrong.

I am a complete novice i am afraid.

//First we start a session
session_start();

//We then check if the user has clicked the login button
if (isset($_POST['submit'])) {

    //Then we include the database connection
    include_once 'dbh.inc.php';
    //And we get the data from the login form
    $pwd = $_POST['pwd'];

    //Error handlers
    //Error handlers are important to avoid any mistakes the user might have made when filling out the form!
    //Check if inputs are empty
    if (empty($pwd)) {
        header("Location: ../index.php?login=empty");
        exit();
    }
    else {
        //Check if username exists in the database USING PREPARED STATEMENTS
        $sql = "SELECT * FROM users WHERE user_uid=?";
        //Create a prepared statement
        $stmt = mysqli_stmt_init($conn);
        //Check if prepared statement fails
        if(!mysqli_stmt_prepare($stmt, $sql)) {
            header("Location: ../index.php?login=error");
            exit();
        }
        //If the prepared statement didn't fail, then continue
        else {
            //Bind parameters/data to the placeholder (?) in our $sql
            mysqli_stmt_bind_param($stmt, "s", $uid);

            //Run query in database
            mysqli_stmt_execute($stmt);

            //Get results from query
      $result = mysqli_stmt_get_result($stmt);

            //If we had a result, which means the username does exist, then assign the database row data to $row.
            if ($row = mysqli_fetch_assoc($result)) {
                //De-hashing the password using the password provided by the user, and the password from the database, to see if they match.
                $hashedPwdCheck = password_verify($pwd, $row['user_pwd']);
                //If they didn't match!
                if ($hashedPwdCheck == false) {
                    header("Location: ../index.php?login=error");
                    exit();
                }
                //If they did match!
                elseif ($hashedPwdCheck == true) {
                    //Set SESSION variables and log user in
                    $_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_uid'];
                    header("Location: ../index.php?login=success");
                    exit();
                }
      } else {
        header("Location: ../index.php?login=error");
            exit();
      }
        }
    }

    //Close the prepared statement
    mysqli_stmt_close($stmt);

} else {
    header("Location: ../index.php?login=error");
    exit();
}