是否可以在 HTML 或 PHP 的输入框中存储变量

Question

I would like to create an input box that is only readable and have the title of the book, is it possible that the input box can store a variable?

PS The one I want to change is the id, now I successfully disabled the input box, but the output is $id instead of the variable that I use the $_GET method.

My code is as follow

<?php
  include_once 'header.php';
    $id = mysqli_real_escape_string($conn, $_GET['bookid']);
    $title = mysqli_real_escape_string($conn, $_GET['title']);
?>

<section class="signup-form">
  <h2>Pre-order</h2>
  <div class="signup-form-form">
    <form action="preorder.inc.php" method="post">
        <input type="text" disabled="disabled" name="id" value= $id >
      <input type="text" name="uid" placeholder="Username...">
      <input type="text" name="BRO" placeholder="BookRegistrationCode...">
      <button type="submit" name="submit">Pre-order</button>
    </form>
  </div>
  <?php
    // Error messages
     if (isset($_GET["error"])) {
      if ($_GET["error"] == "emptyinput") {
        echo "<p>Fill in all fields!</p>";
      }
      else if ($_GET["error"] == "wronginfo") {
        echo "<p>Wrong information!</p>";
      }
        else if ($_GET["error"] == "stmtfailed") {
        echo "<p>Something went wrong!</p>";
      }
      else if ($_GET["error"] == "none") {
        echo "<p>Success!</p>";
      }
     }
  ?>
</section>

Answer 1

Put <?php echo $var; ?> <?php echo $var; ?> inside the value attribute.

<input type="text" value="<?php echo $var; ?>" />

EDIT

As per @arkascha's comment, you can use alternative php short tags:

<input type="text" value="<?= $var; ?>" />

As per the docs:https://www.php.net/manual/en/language.basic-syntax.phptags.php