转换png图像后。 显示一个黑色边框

This is the converted image:

I am trying to convert one PNG image to another PNG image. After image conversion, one black border was created automatically. I don't want the black color border.

This is my code:

$black = imagecolorallocate($thumb, 0, 0, 0); 
imagecolortransparent($thumb, $black); imagepng( $thumb, $path, 1);