create captcha image in phalcon OR how to convert text to image with phalcon?

Hello

how to create captcha image like this in phalcon:

// Create a 100*30 image
$im = imagecreate(100, 30);

// White background and blue text
$bg = imagecolorallocate($im, 255, 255, 255);
$textcolor = imagecolorallocate($im, 0, 0, 255);

// Write the string at the top left
imagestring($im, 5, 0, 0, 'Hello world!', $textcolor);

// Output the image
header('Content-type: image/png');

// 

imagepng($im);
imagedestroy($im);

when use this code in phalcon framework have one error :

cannot be displayed because it contain error

OR how to convert text to image with phalcon????