Login System Using Php And MySql Database

<?php
    require_once("connection.php");

    if($_SERVER['REQUEST_METHOD'] == 'POST'){
        $userName = $_POST["username"];
        $passWord = $_POST["password"];

        $stmt = $connect->prepare("SELECT username,password FROM users_table
        WHERE username = '$userName' AND password = '$passWord'
        ");
        $stmt->execute(array($userName,$passWord));

        $count = $stmt->rowCount();
        if($count > 0){
            session_start();
            $_SESSION["username"] = $userName;
            $_SESSION["password"] = $passWord;

            echo 'welcome' . '<br />' . $_SESSION["username"] . "<br />";
            echo "<a href='logout.php'>Log Out</a>";
        }else{
            echo 'Error';
        }
    }