GD library and resize function

Hey, i think you want to have resize while preserving aspect ratio of the thumbnail. If so, you can try with this helper function:

    function makeThumbnail($data)
    {
        $image = new \Phalcon\Image\Adapter\GD($data['path'] . $data['name']);
        $dimensions = explode('x', $data['size']);
        $new_width = $dimensions[0];
        $new_height = $dimensions[1];

        $source_height = $image->getHeight();
        $source_width = $image->getWidth();
        $source_aspect_ratio = $source_width / $source_height;
        $desired_aspect_ratio = $new_width / $new_height;
        if ($source_aspect_ratio > $desired_aspect_ratio) {
            $temp_width = (int) ($new_height * $source_aspect_ratio);
            $temp_height = $new_height;
        } else {
            $temp_width = $new_width;
            $temp_height = (int) ($new_width / $source_aspect_ratio);
        }
        $x0 = ($temp_width - $new_width) / 2;
        $y0 = ($temp_height - $new_height) / 2;
        $image->resize($temp_width, $temp_height)->crop($new_width, $new_height, $x0, $y0);
        $image->save($data['path'] . $data['size'] . '_' . $data['name']);
    }    

Usage example:

        $data = array(
            'path' => 'PATH_TO_YOUR_IMAGE',
            'name' => 'samnpleimages.jpg',
            'size' => '300x180', 
        );
        $this->makeThumbnail($data);

This will produce thumbnail in same directory as original image with the name:

300x180_samnpleimages.jpg

Could you tell me why should I do it with your code and why does it not work with my code?

Resize function resizes the image to either X or Y axis depending which is bigger. So if you have 1000x2000 image the thumbnail produced by your code will be 180px height and the width will be proportunally calculated but will not be 300.

My code calculates the aspect ratio so it doesnt stretch the image, centers the image in 300x180 canvas and crops whats outside.

You can try liquidRescale from the docs here https://docs.phalcon.io/en/master/api/Phalcon_Image_Adapter_GD.html but you need Imagick.