Use barcode in plugin

Hello guys,
i an trying to generate barcodes using the following script: BarcodePHP

Also, i am creating my plugin using the WordPress Plugin Boilerplate

Heres my code(inside the public plugin folder):

class Coupons {
...
private function __construct() {
...
add_shortcode( 'coupons', array ($this, 'coupons_shortcode') );
..
}

public function coupons_shortcode() {
// Including all required classes
require_once('/includes/barcode/class/BCGArgumentException.php');
require_once('/includes/barcode/class/BCGBarcode.php');
require_once('/includes/barcode/class/BCGBarcode1D.php');
require_once('/includes/barcode/class/BCGFontFile.php');
require_once('/includes/barcode/class/BCGFontFile.php');
require_once('/includes/barcode/class/BCGColor.php');
require_once('/includes/barcode/class/BCGDrawing.php');
require_once('/includes/barcode/class/BCGFontPhp.php');

// Including the barcode technology
require_once('/includes/barcode/class/BCGcode39.barcode.php');

// Loading Font
$font = new BCGFontFile('./includes/barcode/font/Arial.ttf', 18);

// Don't forget to sanitize user inputs
$text = isset($_GET['text']) ? $_GET['text'] : 'HELLO';

// The arguments are R, G, B for color.
$color_black = new BCGColor(0, 0, 0);
$color_white = new BCGColor(255, 255, 255);

$drawException = null;
try {
	$code = new BCGcode39();
	$code->setScale(2); // Resolution
	$code->setThickness(30); // Thickness
	$code->setForegroundColor($color_black); // Color of bars
	$code->setBackgroundColor($color_white); // Color of spaces
	$code->setFont(new BCGFontPhp(5)); // Font (or 0)
	$code->parse($text); // Text
} catch(Exception $exception) {
	$drawException = $exception;
}

 /* Here is the list of the arguments
		// 1 - Filename (empty : display on screen)
		// 2 - Background color */
	$drawing = new BCGDrawing('', $color_white);
	if($drawException) {
		$drawing->drawException($drawException);
	} else {
		$drawing->setBarcode($code);
		$drawing->draw();
	}

// Header that says it is an image (remove it if you save the barcode to a file)
	header('Content-Type: image/png');
	header('Content-Disposition: inline; filename="barcode.png"');

	// Draw (or save) the image into PNG format.
	$drawing->finish(BCGDrawing::IMG_FORMAT_PNG);

	}

}

I am using the [coupons] shortcode inside a page, but when i run the code, i do not get the Barcode, insted i get a message that “The image http://localhost/wordpress/?page_id=2 contains errors”.

The following code inside the coupons_shortcode, is the same as the example provided by the barcodephp example.

I am running this code localy, and i have enabled the gd2 library.

Has anyone tried this before?
Can you please help me?

[Moderator Note: No bumping. If it’s that urgent, consider hiring someone.]