如何允许用户使用php中的用户名和电子邮件登录？

我目前在允许用户使用用户名或电子邮件登录并输入并将其与数据库匹配时遇到问题，这是我的代码：

<?php
//Login Script
if (isset($_POST["user_login"]) && isset($_POST["password_login"])) {
    $user_login = $_POST["user_login"]; 
    $password_login = $_POST["password_login"];
    $md5password_login = md5($password_login);
    $sql = mysqli_query($conn, "SELECT id FROM users2 WHERE username='$user_login' AND password='$md5password_login' LIMIT 1"); // query the person

    //Check for their existance
    $userCount = mysqli_num_rows($sql); //Count the number of rows returned

    if ($userCount == 1) {
        while($row = mysqli_fetch_array($sql,MYSQLI_ASSOC)) {
             $rahul = $row["id"];
        }

        $_SESSION["id"] = $rahul;
        $_SESSION["user_login"] = $user_login;
        $_SESSION["password_login"] = $password_login;
       exit("<meta http-equiv=\"refresh\" content=\"0; url=./home.php\">");
    } else {
        echo 'That information is incorrect, try again';
        exit();
    }
}
?>

我不知道要在mysqli_query中添加什么以允许用户从用户名和电子邮件登录。