PHP准备语句/绑定参数代码崩溃

有人能解释一下为什么这会给我一个500内部服务器错误吗?我试着添加一些sql注入保护，我不确定我做错了什么。我应该用面向对象的方式而不是过程式的方式来做这件事吗?

<?php
$conn = mysqli_connect($host, $user, $pwd)or die("Error connecting to database.");
mysqli_select_db($conn, $db) or die("Couldn't select the database."); 
$username = $_POST['username'];
$password = $_POST['password'];
$stmt = mysqli_stmt_init($conn);
$query = "SELECT * FROM Users WHERE email=? AND password=?";
mysqli_stmt_prepare($stmt, $query) or die("Failed to prepare statement.");
mysqli_stmt_bind_param($stmt, "ss", $username, $password);
mysqli_stmt_execute($stmt);
$result = mysqli_stmt_get_result($stmt);
$count = mysqli_num_rows($result);
if($count == 1){
    //Log in successful 
}
else {
    //Wrong Username or Password        
}
mysqli_close($conn);
?>