<form name="formvideo" action="" method="post">
              Put the URL video here<br>
              <input name="subject" type="text" id="subject" style="width: 800px;" value="">
              <br>
              <input type="submit" value="Submit">
            </form><br>

            <?php
            if (isset($_POST["submit"])){
                $video = $_POST['subject'];
                if ($video == true){
                    echo "success";
                } else {
                echo "fail";
            }
            }

<?php
            if (isset( $_POST['subject'])) {
            echo $_POST['subject'];
            }
            ?>