在PHP中删除图像的背景颜色

I want to remove background color of an image. But it's not working properly. My code can remove background color partially.

Here is my code :

function transparent_background($filename) 
{

    $output_file_name = $_SERVER['DOCUMENT_ROOT'].'/test/removeBackgroundColor/'.rand(100,200).'.png';
    $_filename= $_SERVER['DOCUMENT_ROOT'].'/test/removeBackgroundColor/'.$filename;
    $_backgroundColour='0,0,0';
    $imageData =  file_get_contents($_filename);
    $_img = imagecreatefromstring($imageData);
    $_backgroundColours = explode(',', $_backgroundColour);
    $_removeColour = imagecolorallocate($_img, (int)$_backgroundColours[0], (int)$_backgroundColours[1], (int)$_backgroundColours[2]);
    imagecolortransparent($_img, $_removeColour);
    imagesavealpha($_img, true);


    $_transColor = imagecolorallocatealpha($_img, 0, 0, 0, 127);
    imagefill($_img, 0, 0, $_transColor);
    imagesavealpha($_img, true);
    imagepng($_img, $output_file_name);

    echo $output_file_name;
}

transparent_background('6.png');

I have passed this image as input : http://prntscr.com/nldb94

But it return this : http://prntscr.com/nldbg4

I want to remove this section's color also : http://prntscr.com/nldbr2

Can you please help me to figure out the solution ? Thanks in advance

$_transColor = imagecolorallocatealpha($_img, 0, 0, 0, 127);
    imagefill($_img, 0, 0, $_transColor);
    imagesavealpha($_img, true);
    imagepng($_img, $output_file_name);

Use imagecolortransparent($_img, $_removeColour); in the above code.

So please try including the imagecolortransparent code in the second image code also.