login - index.php

<?php
    $conn = mysqli_connect("127.0.0.1", "root", "123123", "login");
    if(!$conn){
        echo "database connection error";
    }

    session_start();

    if(isset($_POST["login"])){
        $username = mysqli_real_escape_string($conn, $_POST["username"]);
        $password = mysqli_real_escape_string($conn, $_POST["password"]);

        $check_username = mysqli_num_rows(mysqli_query($conn, "SELECT null FROM users WHERE username = '$username'"));
        if($check_username == 0){
            echo "Username not exist";
        }
        else{
            $user_info = mysqli_fetch_assoc(mysqli_query($conn, "SELECT id, password FROM users WHERE username = '$username'"));
            if(password_verify($password, $user_info["password"])){
                $_SESSION["id"] = $user_info["id"];
                header("Location: home.php");
            }
            else{
                echo "Password incorrect";
            }
        }
    }

?>

<form method="post">
    Username: <input name="username" type="text"><br>
    Password: <input name="password" type="password"><br>
    <button name="login" type="submit">Login</button>
</form>

<a href="register.php">Register</a>