PHP GD resizing, upscaling

gdresize

function generate_image_thumbnail($source_image_path, $thumbnail_image_path, $max_width, $max_height, $quality=35) {
    list($source_image_width, $source_image_height, $source_image_type) = getimagesize($source_image_path);
    switch ($source_image_type) {
        case IMAGETYPE_GIF:
            $source_gd_image = imagecreatefromgif($source_image_path);
            break;
        case IMAGETYPE_JPEG:
            $source_gd_image = imagecreatefromjpeg($source_image_path);
            break;
        case IMAGETYPE_PNG:
            $source_gd_image = imagecreatefrompng($source_image_path);
            break;
    }
    if ($source_gd_image === false) {
        return false;
    }
    $source_aspect_ratio = $source_image_width / $source_image_height;
    $thumbnail_aspect_ratio = $max_width / $max_height;
    

    if ($thumbnail_aspect_ratio > $source_aspect_ratio) {
        $thumbnail_image_width = (int) ($max_height * $source_aspect_ratio);
        $thumbnail_image_height = $max_height;
    } else {
        $thumbnail_image_width = $max_width;
        $thumbnail_image_height = (int) ($max_width / $source_aspect_ratio);
    }

    $thumbnail_gd_image = imagecreatetruecolor($thumbnail_image_width, $thumbnail_image_height);
    imagecopyresampled($thumbnail_gd_image, $source_gd_image, 0, 0, 0, 0, $thumbnail_image_width, $thumbnail_image_height, $source_image_width, $source_image_height);
    imagejpeg($thumbnail_gd_image, $thumbnail_image_path, $quality);
    imagedestroy($source_gd_image);
    imagedestroy($thumbnail_gd_image);
    return true;
}

for ($a=25; $a <= 50; $a++) {
  $f = "resize/resize".$a.".jpg";
	generate_image_thumbnail("bigimage.jpg",$f,1280,1280,$a);
	echo '<p><img src="'.$f.'" width="640" />'.$a.'</p>';
}