Create a transparent png file using PHP

Currently I would like to create a transparent png with the lowest quality .

The code:

<?php
function createImg ($src, $dst, $width, $height, $quality) {
    $newImage = imagecreatetruecolor($width,$height);
    $source = imagecreatefrompng($src); //imagecreatefrompng() returns an image identifier representing the image obtained from the given filename.
    imagecopyresampled($newImage,$source,0,0,0,0,$width,$height,$width,$height);
    imagepng($newImage,$dst,$quality);      //imagepng() creates a PNG file from the given image. 
    return $dst;
}

createImg ('test.png','test.png','1920','1080','1');
?>

However, there are some problems:

1. Do I need to specific a png file before creating any new file? Or can I create without any existing png file?

   Warning: imagecreatefrompng(test.png): failed to open stream: No such file or directory in

   C:\DSPadmin\DEV\ajax_optipng1.5\create.php on line 4

2. Although there are error message , it still generate a png file , however, what I found that is the file is a black color image , do I need to specific any parameter to make it transparent?

Thanks.

To 1) imagecreatefrompng('test.png') tries to open the file test.png which then can be edited with GD functions.

To 2) To enable saving of the alpha channel imagesavealpha($img, true); is used. The following code creates a 200x200px sized transparent image by enabling alpha saving and filling it with transparency.

<?php
$img = imagecreatetruecolor(200, 200);
imagesavealpha($img, true);
$color = imagecolorallocatealpha($img, 0, 0, 0, 127);
imagefill($img, 0, 0, $color);
imagepng($img, 'test.png');