HTTP ERROR 500

<?php
session_start();
 
if(isset($_SESSION["loggedin"]) && $_SESSION["loggedin"] === true){
    header("location: admin.php");
    exit;
}
 
$link = mysqli_connect("*","*", "*", "*","*") ;
 
$username = $password = "";
$username_err = $password_err = "";
 
if($_SERVER["REQUEST_METHOD"] == "POST"){
 
    if(empty(trim($_POST["username"]))){
        $username_err = "Zadejte jméno.";
    } else{
        $username = trim($_POST["username"]);
    }
    
    if(empty(trim($_POST["password"]))){
        $password_err = "Zadejte heslo";
    } else{
        $password = trim($_POST["password"]);
    }
    
    if(empty($username_err) && empty($password_err)){
        $sql = "SELECT id, username, password FROM users WHERE username = ?";
        
        if($stmt = $link->prepare($sql)){
            $stmt->bind_param("s", $param_username);
            
            $param_username = $username;
            
            if($stmt->execute()){
                $stmt->store_result();
                
                if($stmt->num_rows == 1){                    
                    $stmt->bind_result($id, $username, $hashed_password);
                    if($stmt->fetch()){
                        if(password_verify($password, $hashed_password)){
                            session_start();
                            
                            $_SESSION["loggedin"] = true;
                            $_SESSION["id"] = $id;
                            $_SESSION["username"] = $username;                            
                            
                            header("location: admin.php");
                        } else{
                            $password_err = "Heslo není správně";
                        }
                    }
                } else{
                    $username_err = "Tento uživatel neexistuje.";
                }
            } else{
                echo "Něco se pokazilo, zkuste to příště";
            }
        }
        
        $stmt->close();
    }
    
    $link->close();
}
?>