how to make a file based user login page

this thing really blown my mind up i want to design a page with a text box where my clients can enter their username and by using php i want to tell the page to check if there is a username such as that in a file named locationn.html if existed create a cookie and let him in another page if not add a new line to the file containing the user name entered.

this is my code in this code "unamec" is the name of the cookie and "$user" is the user name and "umname" is the name of the username text box which its value is sent to the page itself using a post method.

<?php
if(isset($_POST["uname"])){
$user=$_POST["uname"];
$pass=$_POST["passs"];
$see=file_get_contents("locationn.html");
$lines=explode("\n",$see);
foreach($lines as $line){
if($line == $user){
setcookie("unamec",$user,time()+86400,"/");
echo '<script>window.location="main.html";</script>';
}
}
}
?>

I can help you with the storage and retrieval of a username from a file. You could adapt this and couple with session management, to achieve your aims.

<?php
if($_SERVER['REQUEST_METHOD'] == 'POST') {
    $username = isset($_POST['username']) ? $_POST['username'] : null;
    if($username !== preg_replace('/[^a-zA-Z0-9]+/', '', $username))
        throw new Exception('Invalid username.');
    $userStore = new UserStore('/tmp/creds.txt');
    if(! $userStore->isStored($username)) {
        if($userStore->storeUsername($username)) {
            echo $username . ' stored.';
        }
    } else {
        echo $username . ' is on file.';
    }
}


class userStore
{
    public $fp;
    public $filename;

    public function __construct($filename)
    {
        $this->filename = $filename;
        if(!file_exists($this->filename))
            file_put_contents($this->filename, null);
        $this->fp = fopen($this->filename, "r+");
    }

    public function isStored($username) {
        $username_exists = false;

        if(! $size = filesize($this->filename))
            return false;
        $contents  = fread($this->fp, $size);

        $lines = array_filter(explode("\n", $contents));
        foreach($lines as $line) {
            if(trim($line) == $username) {
                $username_exists = true;
                break;
            }
        }

        return $username_exists;
    }

    public function storeUsername($username)
    {
        $fp = $this->fp;
        if (flock($fp, LOCK_EX)) {
            fwrite($fp, "$username\n");
            fflush($fp);
            flock($fp, LOCK_UN);
        } else {
            return false;
        }

        return true;
    }
}
?>
<form method='POST'>
    <input type='text' name='username'>
    <input type='submit' value='login'>
</form>