PHP - Image Captcha: "bug"

<form method="POST" action="traiter.php">
<h3>Quel est le chiffre ?</h3>
<img src="captcha.php" style="70px"/><br>
<input type="text" name="captcha" style="70px" />
<input type="submit" />
</form>

<?php
 session_start();
 $_SESSION['captcha'] = rand (1000,9999);
 $img = imagecreatetruecolor(70, 30);
 $fill_color=imagecolorallocate($img,255,255,255);
 imagefilledrectangle($img, 0, 0, 70, 30, $fill_color);
 $text_color=imagecolorallocate($img,10,10,10);
 $font = "./monofont.ttf";
 imagettftext($img, 23, 0, 5, 30, $text_color, $font, $_SESSION['captcha']);
 header("Content-type: image/jpg");
 imagejpeg($img);
 imagedestroy($img);
 ?>

<?php
if ($_POST['captcha']){
  if ($_POST['captcha']==$_SESSION['captcha']){
	echo 'Bravo !';
  }
  else{
    echo 'Dommage !';
  }
 }
?>