PHP - Login To Sbs 2003 Website

<form method=post>
<input type="hidden" name="action" value="logon">
<table border=0>
<tr>
<td>Username:</td>
<td><input name="username" type="text" size=30></td>
</tr>
<tr>
<td>Password:</td>
<td><input name="password" type="password" size=30></td>
</tr>
<td></td>
<td align="left"><input type=submit value="Sign In"></td>
</tr>
<tr>
<td align="center" colspan=2><font size=-1>Don't have an Account ?</font> <a href="?action=newuser"><font size=-1 color="#0000EE">Sign UP Now !</font></a></td>
</tr>
</table>

<?php
$username="username"; 
$password="password"; 
$url="http://www.example.com/index.php"; 

$postdata = "username=".$username."&password=".$password;

$ch = curl_init(); 
curl_setopt ($ch, CURLOPT_URL, $url); 
curl_setopt ($ch, CURLOPT_SSL_VERIFYPEER, FALSE); 
curl_setopt ($ch, CURLOPT_USERAGENT, "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.6) Gecko/20070725 Firefox/2.0.0.6"); 
curl_setopt ($ch, CURLOPT_TIMEOUT, 60); 
curl_setopt ($ch, CURLOPT_FOLLOWLOCATION, 1); 
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1); 
curl_setopt ($ch, CURLOPT_REFERER, $url); 
curl_setopt ($ch, CURLOPT_POSTFIELDS, $postdata); 
curl_setopt ($ch, CURLOPT_POST, 1); 
$result = curl_exec ($ch); 
header('Location: track.html'); 
//echo $result;  
curl_close($ch);
?>

How to add the ability to login with username or email for login?

<?php 
ob_start();
include('../header.php');
include_once("../db_connect.php");
session_start();
if(isset($_SESSION['user_id'])!="") {
	header("Location: ../dashboard");
}
if (isset($_POST['login'])) {
	$email = mysqli_real_escape_string($conn, $_POST['email']);
	$password = mysqli_real_escape_string($conn, $_POST['password']);
	$result = mysqli_query($conn, "SELECT * FROM users WHERE email = '" . $email. "' and pass = '" . md5($password). "'");
	if ($row = mysqli_fetch_array($result)) {
		$_SESSION['user_id'] = $row['uid'];
		$_SESSION['user_name'] = $row['user'];	
		$_SESSION['user_email'] = $row['email'];		
		header("Location: ../dashboard");
	} else {
		$error_message = "Incorrect Email or Password!!!";
	}
}
?>

Hello, I am once again desperately asking for your help,

I am working on a simple login page and I am having trouble actually getting it  to login. I display error messages for if the user doesn't enter anything but I can't seem to get it to work for if the credentials are wrong. It logs the user in whether the information is right or not and i dont even know what to do now

This is the code any suggestions would be greatly appreciated

<?php
    /*
    Name: Deanna Slotegraaf 
    Course Code: WEBD3201
    Date: 2020-09-22
    */

    $file = "sign-in.php";
    $date = "2020-09-22";
    $title = "WEBD3201 Login Page";
    $description = "This page was created for WEBD3201 as a login page for a real estate website";
    $banner = "Login Page";
    require 'header.php';

    $error = "";

    if($_SERVER["REQUEST_METHOD"] == "GET")
    {
        $username = "";
        $password = "";
        $lastaccess = "";        
        $error = "";
        $result = "";
        $validUser = "";
    }
    else if($_SERVER["REQUEST_METHOD"] == "POST")
    {        
    	$conn;
        $username = trim($_POST['username']); //Remove trailing white space
        $password = trim($_POST['password']); //Remove trailing white space

    if (!isset($username) || $username == "") {
      $error .= "<br/>Username is required";
    }
    if (!isset($password) || $password == ""){
      $error .= "<br/>Password is required";
    }
    if ($error == "") {
      $password = md5($password);
      $query = "SELECT * FROM users WHERE EmailAddress='$username' AND Password='$password'";
      $results = pg_query($conn, $query);
      //$_SESSION['username'] = $username;
        //$_SESSION['success'] = "You are now logged in";
        header('location: dashboard.php');
      }else {
        $error .= "Username and/or Password is incorrect";
      }

}

?>

<div class = "form-signin">
    <?php
             echo "<h2 style='color:red; font-size:20px'>".$error."</h2>";
    ?>
<form action = "<?php echo $_SERVER['PHP_SELF'];  ?>" method="post">

      <label for="uname"><b>Login ID</b></label>
      <input type="text" name="username" value="<?php echo $username; ?>"/>
      <br/>
      <label for="psw"><b>Password</b></label>
      <input type="password" name="password" value="<?php echo $password; ?>"/>
      <br/>
        <button type="submit" name="login_user">Login</button>
        <button type="reset">Reset</button></div>
</form>
</div>

<?php require "footer.php"; ?>