"> "> "> 
get.php

<!DOCTYPE html>
<html>
  <head>
    <title></title>
  </head>
  <body>
    <div>GET test</div>
    <form action="get_result.php" method="get">
      <input type="text" name="id" placeholder="Enter id here">
      <input type="text" name="age" placeholder="Enter age here">
      <input type="submit" value="Submit">
    </form>
  </body>
</html>

get_result.php

<?php
  $id = $_GET['id'];
  $age = $_GET['age'];

    if(!$id)
  {
    echo "<script>alert('로그인 후 이용해주세요.');window.close();</script>";
    exit;
  }
?>

<!DOCTYPE html>
<html>
  <head></head>
  <body>
    <p>ID: <?php echo $id; ?></p>
    <p>Age: <?php echo $age; ?></p>
  </body>
</html>