use https only for login page not the whole website

Question

i want to open login page of my website with https only,not the comeplete website. after login authenication ( successful), website whoud again run on http.

currently my main login page is test_index.php where i included test_header.php

my basic code on test_header.php is

if($_SERVER['SERVER_PORT'] != 443) {
   header("HTTP/1.1 301 Moved Permanently");
   header("Location: https://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']);
   exit();
}

but this make complete website in https
i also read here that it can be possible via .htaccess, so i remove above code snippet from test_header.php and add the following lines in .htaccess file and

<IfModule mod_rewrite.c>
  RewriteEngine on
  # 301 redirect to domain to 'www.'
  RewriteCond %{HTTP_HOST} ^testweb.com$ [NC]
  RewriteRule ^(.*)$ http://www.testweb.com/$1 [R=301,L]
</IfModule>

<FilesMatch test_index.php>
RewriteEngine on
RewriteCond %{HTTPS} !=on
RewriteRule .* https://%{SERVER_NAME}%{REQUEST_URI} [R,L]
</FilesMatch>

Note: testweb.com is just an imaginary name, not actual website

but still complete website run on https, please tell me where i m doing mistake??

Edit

@webbiedave please check my updated Code , is that right way??

if ($_SERVER['REQUEST_URI'] == '/test_index.php') { // only check https on login
    if (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == 'on') {
        header("HTTP/1.1 301 Moved Permanently");
        header("Location: https://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']);
        exit();
    } else {
        die("Sorry,Your website is not secure");        
    }
} elseif (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == 'on') {
        // header("HTTP/1.1 301 Moved Permanently");
        header("Location: http://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']);
        exit();
}

Thanks