Cherche Math Captcha

Si non, j'ai ceci qui fonctionne bien mais sur un site qui n'est pas CakePHP.
J'ai mis ce fichier dans /app/lib, mais apparemment, il y a un probleme au moement de la génération de l'image

J'aimerais bien adapter ce fichier pour faire une bonne petit lib pour Cake

<?php
class Capthca{

    #session_start();

    /*===============================================================
        General captcha settings
      ===============================================================*/
    // captcha width
    $captcha_w = 150;
    // captcha height
    $captcha_h = 35 ;
    // minimum font size; each operation element changes size
    $min_font_size = 12;
    // maximum font size
    $max_font_size = 18;
    // rotation angle
    $angle = 20;
    // background grid size
    $bg_size = 13;
    // path to font - needed to display the operation elements
    $font_path = 'fonts/courbd.ttf';
    // array of possible operators
    $operators=array('+','-','*');
    // first number random value; keep it lower than $second_num
    $first_num = rand(1,5);
    // second number random value
    $second_num = rand(6,11);

    /*===============================================================
        From here on you may leave the code intact unless you want
        or need to make it specific changes. 
      ===============================================================*/

    shuffle($operators);
    $expression = $second_num.$operators[0].$first_num;
    #echo "=";
    /*
        operation result is stored in $session_var
    */
    eval("\$session_var=".$second_num.$operators[0].$first_num.";");
    /* 
        save the operation result in session to make verifications
    */
    $_SESSION'security_number'] = $session_var;
    /*
        start the captcha image
    */

    $img = imagecreate( $captcha_w, $captcha_h );
    /*
        Some colors. Text is $black, background is $white, grid is $grey
    */
    $black = imagecolorallocate($img,0,0,0);
    $white = imagecolorallocate($img,255,255,255);
    $grey = imagecolorallocate($img,215,215,215);
    /*
        make the background white
    */
    imagefill( $img, 0, 0, $white );    
    /* the background grid lines - vertical lines */
    for ($t = $bg_size; $t<$captcha_w; $t+=$bg_size){
        imageline($img, $t, 0, $t, $captcha_h, $grey);
    }
    /* background grid - horizontal lines */
    for ($t = $bg_size; $t<$captcha_h; $t+=$bg_size){
        imageline($img, 0, $t, $captcha_w, $t, $grey);
    }

    /* 
        this determinates the available space for each operation element 
        it's used to position each element on the image so that they don't overlap
    */
    $item_space = $captcha_w/3;

    /* first number */
    imagettftext(
        $img,
        rand(
            $min_font_size,
            $max_font_size
        ),
        rand( -$angle , $angle ),
        rand( 10, $item_space-20 ),
        rand( 25, $captcha_h-25 ),
        $black,
        $font_path,
        $second_num);

    /* operator */
    imagettftext(
        $img,
        rand(
            $min_font_size,
            $max_font_size
        ),
        rand( -$angle, $angle ),
        rand( $item_space, 2*$item_space-20 ),
        rand( 25, $captcha_h-25 ),
        $black,
        $font_path,
        $operators[0]);

    /* second number */
    imagettftext(
        $img,
        rand(
            $min_font_size,
            $max_font_size
        ),
        rand( -$angle, $angle ),
        rand( 2*$item_space, 3*$item_space-20),
        rand( 25, $captcha_h-25 ),
        $black,
        $font_path,
        $first_num);

        /* image is .jpg */
    header("Content-type:image/jpeg");
    /* name is secure.jpg */
    header("Content-Disposition:inline ; filename=secure.jpg");
    /* output image */
    imagejpeg($img);
}

?>

PS : Mon application est en local sur mon ordinateur portable (MAMP)