Session variable not the same

Hi all,
I have a trouble when i implement simple captcha for my contact page.

I created a plugin with two file content.

In the_simple_captcha my code is:

add_action('init','captcha_start',1);
function captcha_start(){
    if(!session_id()){
        session_start();
    }
}
function the_simple_captcha(){
    $captcha = plugins_url('captcha.php',__FILE__);
    echo $_SESSION['captcha']; //test session
?>
    <img src="<?php echo $captcha ?>" alt="the simple captcha" />
<?php
}
?>

and my captcha file

<?php
    $string = substr(md5(rand(0,999)),10,6);
    $_SESSION['captcha'] = $string;
    $img=imagecreate(100,20);
    imagefill($img,0,0,imagecolorallocate($img,109,177,228));
    imagestring($img,5,20,3,$_SESSION['captcha'],imagecolorallocate($img,0,0,0));
    header("Content-type: image/png");
    imagepng($img);
    imagedestroy($img);
?>

But $_SESSION[‘captcha’] not same image captcha.