<?php
session_start();
require 'conn.php';

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

    $uname = $_POST['username'];
    $pwd = $_POST['password'];
    $sql = "SELECT * FROM chat WHERE username = ?";
    $stmt = $db->prepare($sql);
    // you just need to find matching username
    $stmt->bind_param("s", $uname);
    $stmt->execute();
    $result = $stmt->get_result();
    $re = $result->fetch_assoc()
    // check if matching username is found, then verify hashed password from database
    if ($re && password_verify($pwd, $re['password'])) {
        $_SESION['username'] = $re['username'];
        exit(header('location: home.php'));
    } else {
        echo "wrong username/password";
    }
}