hesy

$filename = 'data/output/test.png';

$image = imagecreatefrompng($filename);

$black = imageColorAllocate($image, 0, 0, 0);
 
$size = getimagesize($filename);
$w = $size[0];
$h = $size[1];

for ($x=0; $x < $w; $x++){
    for ($y=0; $y < $h; $y++){
    
        $color = imagecolorat($image, $x, $y);

        $r = ($color >> 16) & 0xFF;
        $g = ($color >> 8) & 0xFF;
        $b = $color & 0xFF;
        
        if ($r > 210 and $g > 210 and $b > 210){
            imagesetpixel($image, $x , $y, $black);
        }
    }
}

imagecolortransparent($image, $black);

$res_path = 'data/output/result.png';

imagepng($image, $res_path, 9, NULL);
imagedestroy($image);