submitting login and sign up forms using AJAX

<?php

session_start();

  // check whether the loginbtn was clicked
if (isset($_POST['loginbtn'])) {

  include_once 'dbh.inc.php';

    // initialize variable
  $uid = trim($_POST['uid']);
  $pwd = trim($_POST['pwd']);

    // error handlers

    // check if inputs are empty
  if (empty($uid) || empty($pwd)) {

    $conn = null;
    header("Location: ../index.php?login=error_field");
    exit();

  } else {

      // check if username is in database
    $stmt = $conn->prepare("SELECT user_uid FROM users WHERE user_uid = ?");
    $stmt->execute([$uid]);

    if ($stmt->rowCount() < 1) {

      $conn = null;
      header("Location: ../index.php?login=error_username");
      exit();

    } else {

        // check if password is correct
      $stmt = $conn->prepare("SELECT user_pwd FROM users WHERE user_uid = ?");
      $stmt->execute([$uid]);
      $result = $stmt->fetchAll(PDO::FETCH_ASSOC);

        // dehashing the password
      $hashedPwdCheck = password_verify($pwd, $result[0]['user_pwd']);

      if ($hashedPwdCheck == false) {

        $conn = null;
        header("Location: ../index.php?login=error_password");
        exit();

      } else {

          // login the user in
        $stmt = $conn->prepare("SELECT user_id, user_first, user_last, user_email, user_uid FROM users WHERE user_uid = ?");
        $stmt->execute([$uid]);
        $result = $stmt->fetchAll(PDO::FETCH_ASSOC);

        $_SESSION['user_id'] = $result[0]['user_id'];
        $_SESSION['user_first'] = $result[0]['user_first'];
        $_SESSION['user_last'] = $result[0]['user_last'];
        $_SESSION['user_email'] = $result[0]['user_email'];
        $_SESSION['user_uid'] = $result[0]['user_uid'];

        $conn = null;
        header("Location: ../updates.php?login=success");
        exit();

      }

    }

  }

} else {

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

}

?>