Problema con CAPTCHA

[GMR_STAFE]

Estoy creando un formulario para crear cuentas y le puse para que pida un CAPTCHA para evitar spam, pero me tira error por una variable. Les dejo el código.

Código:

<?php
  // Insert the page header
  $page_title = 'Sign Up';
  require_once('header.php');

  require_once('appvars.php');
  require_once('connectvars.php');

  // Connect to the database
  $dbc = mysqli_connect(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);

  if (isset($_POST['submit'])) {
    // Grab the profile data from the POST
    $username = mysqli_real_escape_string($dbc, trim($_POST['username']));
    $password1 = mysqli_real_escape_string($dbc, trim($_POST['password1']));
    $password2 = mysqli_real_escape_string($dbc, trim($_POST['password2']));
	$mail = mysqli_real_escape_string($dbc, trim($_POST['mail']));
	
	// Check the CAPTCHA pass-phrase for verification
    $user_pass_phrase = sha1($_POST['verify']);
    if ($_SESSION['pass_phrase'] == $user_pass_phrase) {

    if (!empty($username) && !empty($password1) && !empty($password2) && ($password1 == $password2) && !empty($mail)){
      // Make sure someone isn't already registered using this username
      $query = "SELECT * FROM mismatch_user WHERE username = '$username'";
      $data = mysqli_query($dbc, $query);
      if (mysqli_num_rows($data) == 0) {
        // The username is unique, so insert the data into the database
        $query = "INSERT INTO mismatch_user (username, password, mail, join_date) VALUES ('$username', SHA('$password1'), '$mail', NOW())";
        mysqli_query($dbc, $query);

        // Confirm success with the user
        echo '<p>Your new account has been successfully created. You\'re now ready to <a href="login.php">log in</a>.</p>';

        mysqli_close($dbc);
        exit();
      }
      else {
        // An account already exists for this username, so display an error message
        echo '<p class="error">An account already exists for this username. Please use a different address.</p>';
        $username = "";
      }
    }
    else {
      echo '<p class="error">You must enter all of the sign-up data, including the desired password twice.</p>';
    }
  }
    else {
      echo '<p class="error">Please enter the verification pass-phrase exactly as shown.</p>';
    }
 }

  mysqli_close($dbc);
?>

  <p>Please enter your username and desired password to sign up to Mismatch.</p>
  <form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
    <fieldset>
      <legend>Registration Info</legend>
      <label for="username">Username:</label>
      <input type="text" id="username" name="username" value="<?php if (!empty($username)) echo $username; ?>" /><br />
      <label for="password1">Password:</label>
      <input type="password" id="password1" name="password1" /><br />
      <label for="password2">Password (retype):</label>
      <input type="password" id="password2" name="password2" /><br />
      <label for="mail">Mail:</label>
      <input type="mail" id="mail" name="mail" /><br />
     <label for="verify">Verification:</label>
    <input type="text" id="verify" name="verify" value="Enter the pass-phrase." /> <img src="captcha.php" alt="Verification pass-phrase" />
    </fieldset>
    <input type="submit" value="Sign Up" name="submit" />
  </form>

<?php
  // Insert the page footer
  require_once('footer.php');
?>

Este es el del CAPTCHA. Lo probé en otro formulario más sencillo y me funcionó bien

Código:

<?php
  session_start();

  // Set some important CAPTCHA constants
  define('CAPTCHA_NUMCHARS', 6);  // number of characters in pass-phrase
  define('CAPTCHA_WIDTH', 100);   // width of image
  define('CAPTCHA_HEIGHT', 25);   // height of image

  // Generate the random pass-phrase
  $pass_phrase = "";
  for ($i = 0; $i < CAPTCHA_NUMCHARS; $i++) {
    $pass_phrase .= chr(rand(97, 122));
  }

  // Store the encrypted pass-phrase in a session variable
  $_SESSION['pass_phrase'] = sha1($pass_phrase);

  // Create the image
  $img = imagecreatetruecolor(CAPTCHA_WIDTH, CAPTCHA_HEIGHT);

  // Set a white background with black text and gray graphics
  $bg_color = imagecolorallocate($img, 255, 255, 255);     // white
  $text_color = imagecolorallocate($img, 0, 0, 0);         // black
  $graphic_color = imagecolorallocate($img, 64, 64, 64);   // dark gray

  // Fill the background
  imagefilledrectangle($img, 0, 0, CAPTCHA_WIDTH, CAPTCHA_HEIGHT, $bg_color);

  // Draw some random lines
  for ($i = 0; $i < 5; $i++) {
    imageline($img, 0, rand() % CAPTCHA_HEIGHT, CAPTCHA_WIDTH, rand() % CAPTCHA_HEIGHT, $graphic_color);
  }

  // Sprinkle in some random dots
  for ($i = 0; $i < 50; $i++) {
    imagesetpixel($img, rand() % CAPTCHA_WIDTH, rand() % CAPTCHA_HEIGHT, $graphic_color);
  }

  // Draw the pass-phrase string
  imagettftext($img, 18, 0, 5, CAPTCHA_HEIGHT - 5, $text_color, 'Courier New Bold.ttf', $pass_phrase);

  // Output the image as a PNG using a header
  header("Content-type: image/png");
  imagepng($img);

  // Clean up
  imagedestroy($img);
?>