How to ensure that cookies are not falsified when using cookies to save logged-in user information

How to ensure that cookies are not forged when using cookies to save logged-in user information
You master. Ask you a question, I would like to use a cookie to save the user login, the purpose is to use the HTML page (display user name and automatically fill in the text box), and to be able to login for a period of time.
Excuse me, please. How can I guarantee that a cookie will not be forged?

------Solution--------------------
Select Remember password to log in when
$spuerpass = "20120412001@#"//self-change, user cannot get
$user _id= $rows _arr[' user_id ';
$rand =MD5 (MD5 ($user _id. $spuerpass));
Setcookie (' User_, $user _id, (Time () +2592000), '/', ', 0);
Setcookie (' Rand ', $rand, (Time () +2592000), '/', ', 0);
$_session[' user_id ']= $user _id;


Next open the page to detect if there is a remember password login
if (Isset ($_cookie[' user_id ') &&isset ($_cookie[' rand '))
{
$user _id=$_cookie[' user_id '];
$spuerpass = "20120412001@#";
$rand =MD5 (MD5 ($user _id. $spuerpass));
if ($_cookie[' rand ']== $rand)
{
$_session[' user_id ']= $user _id;
}
}

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