<?php

$dbServername = "localhost";
$dbUsername = "root";
$dbPassword = "root";
$dbName = "Login System";

$con = mysqli_connect($dbServername, $dbUsername, $dbPassword, $dbName);

<?php

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

    include_once 'dbh.inc.php';

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

    //Error handlers
    //Check for empty fields
    if (empty($first) || empty($last) || empty($email) || empty($uid) || empty($pwd)) {
        echo <h2>Please fill in all fields;
        exit ();
    } else {
        //Check if input characters are valid
        if (!preg_match("/^[a-zA-Z]*$/", $first) || !preg_match("/^[a-zA-Z]*$/", $last)) {
            header("Location: ../signup.php?signup=invalid");
            exit();
        } else {
            //Check if email is valid
            if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
                header("Location: ../signup.php?signup=email");
                exit();
            } else {
                $sql = "SELECT * FROM users WHERE user_uid='$uid'";
                $result = mysqli_query($conn, $sql);
                $resultCheck = mysqli_num_rows($result);

                if ($resultCheck > 0) {
                    header("Location: ../signup.php?signup=usertaken");
                    exit();
                } else {
                    //Hashing the password
                    $hashedPwd = password_hash($pwd, PASSWORD_DEFAULT);
                    //Insert the user into the database
                    $sql = "INSERT INTO users (first, last, email, uid, pwd) VALUES ('$first', '$last', '$email', '$uid', '$hashedPwd');";
                    mysqli_query($conn, $sql);
                    header("Location: ../signup.php?signup=success");
                    exit();
                }
            }
        }
    }

} else {
    header("Location: ../signup.php");
    exit();
}

?>

<?php
    include_once 'header.php';
?>

    <section class="main-container">
        <div class="main-wrapper">
            <h2>Signup</h2>
            <form class="signup-form" action="includes/signup.inc.php" method="POST">
                <input type="text" name="first" placeholder="Firstname">
                <input type="text" name="last" placeholder="Lastname">
                <input type="text" name="email" placeholder="E-mail">
                <input type="text" name="uid" placeholder="Username">
                <input type="password" name="pwd" placeholder="Password">
                <Button type="submit" name="submit">Sign up</Button>
            </form>

        </div>
    </section>

<?php
    include_once 'footer.php';
?>

$conn = mysqli_connect( ... );

if ( !$conn ) {
    die( 'Did not connect: ' . mysqli_connect_error() );
}

$result = mysqli_query( $conn, $sql );

if (false === $result) {
    die( 'Query error: ' . mysqli_error($conn) );
}

<?php
if (isset($_POST['submit'])) {
  include_once 'dbh.inc.php';
  $first = strip_tags(trim($_POST['first']));
  $last = strip_tags(trim($_POST['last']));
  $email = filter_var(trim($_POST['email'], FILTER_SANITIZE_EMAIL));
  $uid = strip_tags(trim($_POST['uid']));
  $pwd = strip_tags(trim($_POST['pwd']));
  //Error handlers
  //Check for empty fields
  if (empty($first) || empty($last) || empty($email) || empty($uid) || empty($pwd)) {
    echo '<h2>Please fill in all fields</h2>'; // was echo <h2>Please fill in all fields; which would cause an error 500
    exit ();
  }
  else {
    //Check if input characters are valid
    if (!preg_match("/^[a-zA-Z]*$/", $first) || !preg_match("/^[a-zA-Z]*$/", $last)) {
      header("Location: ../signup.php?signup=invalid");
      exit();
    }
    else {
      //Check if email is valid
      if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        header("Location: ../signup.php?signup=email");
        exit();
      }
      else {
        $sql = "SELECT * FROM users WHERE user_uid='$uid'";
        $result = mysqli_query($conn, $sql);
        $resultCheck = mysqli_num_rows($result);
        if ($resultCheck > 0) {
          header("Location: ../signup.php?signup=usertaken");
          exit();
        }
        else {
          //Hashing the password
          $options = [
            'cost' => 12,
          ];
          $hashedPwd = password_hash($password, PASSWORD_BCRYPT, $options); // Adding salt to hashed password
          //Insert the user into the database
          $sql = "
                    INSERT INTO users (first, last, email, uid, pwd)
                    VALUES ('" . $first . "',
                    '" . $last . "',
                    '" . $email . "',
                    '" . $uid . "',
                    '" . $hashedPwd . "');";
          mysqli_query($conn, $sql);
          header("Location: ../signup.php?signup=success");
          exit();
        }
      }
    }
  }
}
else {
  header("Location: ../signup.php");
  exit();
}
?>

ALTER TABLE `users` ADD `serial` INT PRIMARY KEY AUTO_INCREMENT;