[Resolvido] Captcha com fonte maiscula & background

Olá caro amigos da iMasters, estou com um pequeno problema, no meu captcha as LETRAS saem minusculas, exemplo: dad6af19 eu gostaria que elas fosse maiscula ex: DAD6AF19

Code Captcha:

<?php
session_start();

class CaptchaSecurityImages {

var $font = 'arial.ttf';

function generateCode($characters) {
	/* list all possible characters, similar looking characters and vowels have been removed */
	$possible = '23456789bcdfghjkmnpqrstvwxyz';
	$code = '';
	$i = 0;
	while ($i < $characters) {
 $code .= substr($possible, mt_rand(0, strlen($possible)-1), 1);
 $i++;
	}
	return $code;
}

function CaptchaSecurityImages($width='170',$height='45',$characters='8') {
	$code = $this->generateCode($characters);
	/* font size will be 75% of the image height */
	$font_size = $height * 0.60;
	$image = @imagecreate($width, $height) or die('Cannot initialize new GD image stream');
	/* set the colours */
	$background_color = imagecolorallocate($image, 255, 255, 255);
	$text_color = imagecolorallocate($image, 0, 0, 0);
	//$noise_color = imagecolorallocate($image, 100, 120, 180);
	/* generate random dots in background */
	for( $i=0; $i<($width*$height)/3; $i++ ) {
 imagefilledellipse($image, mt_rand(0,$width), mt_rand(0,$height), 1, 1, $noise_color);
	}
	/* generate random lines in background */
	for( $i=0; $i<($width*$height)/150; $i++ ) {
 imageline($image, mt_rand(0,$width), mt_rand(0,$height), mt_rand(0,$width), mt_rand(0,$height), $noise_color);
	}
	/* create textbox and add text */
	$textbox = imagettfbbox($font_size, 0, $this->font, $code) or die('Error in imagettfbbox function');
	$x = ($width - $textbox[4])/2;
	$y = ($height - $textbox[5])/2;
	imagettftext($image, $font_size, 0, $x, $y, $text_color, $this->font , $code) or die('Error in imagettftext function');
	/* output captcha image to browser */
	header('Content-Type: image/jpeg');
	imagejpeg($image);
	imagedestroy($image);
	$_SESSION['security_code'] = $code;
}

}

$width = isset($_GET['width']) ? $_GET['width'] : '170';
$height = isset($_GET['height']) ? $_GET['height'] : '45';
$characters = isset($_GET['characters']) && $_GET['characters'] > 1 ? $_GET['characters'] : '8';

$captcha = new CaptchaSecurityImages($width,$height,$characters);

?>

Também gostaria de saber como fasso para colocar um background q tenho que chama: captcha.jpg

Agradeço galera obrigado pela ajuda, vocês são os melhores!!!

///

@edited Consegui resolver o problema usei a função

strtoupper