How to display an error in the authorization form with incorrectly entered data?

Question

PHP

How to finish checking the login and password during authorization (should give an error - if the data is entered incorrectly)?
I have this code:

if(isset($_POST[login_admin]))
{
if($_POST[login] == $admins[login] || md5($_POST[password]) == $admins[password])
{
if(isset($_POST[me])) {$time = '900000000';} else {$time = '84000';}
setcookie('hash', md5($_POST[password]), time()+$time);
setcookie('login',$_POST[login], time()+$time);
header('Location: index.php');
}
}

It is necessary to finish the feature, which would give an error if the username or password is incorrect.
Thanks to all.

Answer 1

if($_POST[login] == $admins[login] || md5($_POST[password]) == $admins[password])

If in this section $_POST[login] == $admins[login] you are comparing login and password md5($_POST[password]) == $admins[password])
then the condition should not be "or" ( || ) , and "and"( && )
i.e. to start doing this:
Well, to the point of the question:

if(isset($_POST[login_admin]))
{
    if($_POST[login] == $admins[login] && md5($_POST[password]) == $admins[password]){
    if(isset($_POST[me])) {$time = '900000000';} else {$time = '84000';}
        setcookie('hash', md5($_POST[password]), time()+$time);
        setcookie('login',$_POST[login], time()+$time);
        header('Location: index.php');
    }else{
        echo 'Error';
    }
}

ps. well, it would be nice to read:
if else
password_hash - md5 is not relevant at all
Validation of data from the user

Answer 2

I can't make an array with errors. Tell me how to do it in this case? *