PHP - Encrypt Password In Login Script

<?php 
$page_title = 'Logged in'; //Dynamic title 
include('C:/wamp/www/PHP/includes/header.html');
?> 
<?php
session_start();
//A array for the sites users with passwords
$users = array(
                'Dexter'=>'meow1',
                'Garfield'=>'meow2',
		'Miro'=>'meow3'
                );

//A handle to save the varible users to file on a new line from the last entry				
$handle = fopen("newusers.txt, \n\r")
$txt = $users;
fclose($handle);


if(isset($_GET['logout'])) {
    $_SESSION['username'] = '';
    header('Location:  ' . $_SERVER['PHP_SELF']);
}

if(isset($_POST['username'])) {
    if($users[$_POST['username']] == $_POST['password']) { 
        $_SESSION['username'] = $_POST['username'];
    }else {
        echo "Something went wrong, Please try again";
    }
}
?>

<?php 
echo "<h3>Login</h3>";
echo "<br />";
?>
<!--A legend form to login-->
		<fieldset><legend>Fill in your username and password</legend>
        <form name="login" action="777log.php" method="post">
            Username: <br /> 
			<input type="text" name="username" value="" /><br />
            Password: <br /> 
			<input type="password" name="password" value="" /><br />
			<br />
            <input type="submit" name="submit" value="Login" />
	</fieldset>
	</form> 

<?php //Footer include file 
include('C:/wamp/www/PHP/includes/footer.html');
?>

<?php //Header
$page_title = 'Reading a file'; 
include('C:/wamp/www/PHP/includes/header.html');
?> 

<?php 
	session_start();
	//Use an array forthe sites users  
$users = array(
                'Dexter'=>'meow1',
                'Garfield'=>'meow2',
		'Miro'=>'meow3'
                );
//
if(isset($_GET['logout'])) {
    $_SESSION['username'] = '';
	echo "You are now loged out";
	//The user is loged out and returned to the login page
    header('Location:  ' . $_SERVER['PHP_SELF']); 
}

if(isset($_POST['username'])) {
	//Something goes wrong here when login without any boxes filled
    if($users[$_POST['username']] == $_POST['password']) { 
        $_SESSION['username'] = $_POST['username'];
    }else {
        echo "Something went wrong, Please try again";
		$redirect = "Location: 777.php";
    }
}
?>
        <?php if($_SESSION['username']): ?>
            <p><h2>Welcome <?=$_SESSION['username']?></h2></p> 
			<p align="right"><a href="777.php">Logga ut</a></p><?php endif; ?>
			<p>Today Ben&Jerrys Chunky Monkey is my favorite!</p>	

<?php //Footer
include('C:/wamp/www/PHP/includes/footer.html');
?>

    /**
     * Select the database
     *
     * @access    private called by the base class
     * @return    resource
     */
    function db_select()
    {
        return @mysql_select_db($this->database, $this->conn_id);
    }

I have developed a code for a login and seems to work well (No syntax error according to https://phpcodechecker.com/ but when I enter a username and a password in the login form, I get an error HTTP 500. I think that everything is ok in the code but obviously there is something that I am not thinking about. The code (excluding db connection):

$id="''"; $username = $_POST['username']; $password = md5($_POST['password']); $func = "SELECT contrasena FROM users WHERE username='$username'";

$realpassask = $conn->query($func); $realpassaskres = $realpassask->fetch_assoc(); $realpass= $realpassaskres[contrasena];

$func2 = "SELECT bloqueado FROM users WHERE username='$username'"; $blockedask = $conn->query($func2); $blockedres = $blockedask->fetch_assoc(); $bloqueado = $blockedres[bloqueado];

//Login if(!empty($username)) { // Check the email with database
$userexists = $pdo->prepare("SELECT COUNT(username) FROM users WHERE username= '$username' LIMIT 1"); $userexists->bindParam(':username', $username); $userexists->execute(); // Get the result $userexistsres = $userexists->fetchColumn(); // Check if result is greater than 0 - user exist if( $userexistsres == 1) { if ($bloqueado == NO) { if ($password != $realpass) { die("contrasena incorrecta"); } Else{ $_SESSION['loguin']="OK"; $_SESSION['username']="$username"; header("Location: ./index.php"); } } Else{ die("Tu usuario ha sido bloqueado o todavía no ha sido aceptado por un administrador. } Else{ die("No hay ninguna cuenta con este nombre de usuario"); } } else { echo 'El campo usuario esta vacio'; } ?>

<?php

error_reporting(E_ALL | E_NOTICE);
ini_set('display_errors', '1');

require 'connect.php';

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

	$newp = trim($_POST['npass']);
	$confp = trim($_POST['cpass']);

	if(empty(trim($newp))) {

		echo "<h3><center>You did not enter a new password!</center></h3>";

		exit();

	} if(empty(trim($confp))) {

		echo "<h3><center>You must confirm the password!</center></h3>";

		exit();

	} if($confp !== $newp) {

		echo "Passwords do not match!, try again.";
	} else {

		$sql = "UPDATE $db_name SET cpass='$password' WHERE id=' ".$row['id']." '";

		echo " ".$row['username']."\s password has been reset! ";
	}
}

?>

<html><title>  Change password  </title><head><style>#form {border-radius: 20px;font-family: sans-serif; margin-top: 60px; padding: 30px;background-color: #aaa;margin-left: auto; margin-right: auto; width: 500px; clear: both;} #form input {width: 100%; clear: both;} #form input:hover {border: 1px solid #ff0000;}</style></head>
<body>
<div id="form">
	<form action='' method='POST'>
	<h2><b><center>Change Password</center></b></h2><br>
	<tr>
	<td><b>New password:</b><input type="password" name="npass" placeholder="Enter new password" /></td><br><br>
	<td><b>Confirm password:</b><input type="password" name="cpass" placeholder="Confirm password" /></td><br><br>
	<td><input type="submit" name="change" value="Change!" /></td>
</tr>
</form>
</div><!-- end of form div -->
</body>
</html>

Notice: Undefined variable: row in C:\xampp\htdocs\Login\web_dir\changepassword.php on line 30

Notice: Undefined variable: row in C:\xampp\htdocs\Login\web_dir\changepassword.php on line 32

\s password has been reset!

<?php

error_reporting(E_ALL | E_NOTICE);
ini_set('display_errors', '1');

session_start();

require 'connect.php';

echo "<title>  Edit a user  </title>";

$sql = "SELECT id, username FROM $tbl_name ORDER BY username";
$result = $con->query($sql);
while ($row = $result->fetch_assoc())
{
   	echo "<div id='l'><tr><td>{$row['username']}</td>&nbsp;|
        <td><a href='editUser.php?id={$row['id']}'>Edit User</a>&nbsp;|</td>
        <td><a href='changepassword.php?id={$row['id']}'>Change Password</a>&nbsp;|</td>
        <td><a href='banUser.php?id={$row['id']}'>Ban User</a></td><br><br>
        </tr></div>\n";
}


?>