Change uploaded image background to transparent color

$img_name = $path;  // 新图片地址
$image = file_get_contents($path);
$info = getimagesize($path);
$im = imagecreatefromstring($image);
$width = $info[0];
$height = $info[1];
for($i=0;$i<$height;$i+=1){
    for($j=0;$j<$width;$j+=1){
        $rgb = ImageColorAt($im, $j, $i);
        $r = ($rgb >> 16) & 0xFF;
        $g = ($rgb >> 8) & 0xFF;
        $b = $rgb & 0xFF;

        if(intval($r)>140 && $g >140 && $b>140){
            $hex = imagecolorallocate($im, 255, 255, 255);
            imagesetpixel($im,$j, $i, $hex);
        }
    }
}
$white = imagecolorallocate($im , 255 , 255 ,255);//Pick up white
imagefill($im, 0, 0, $white);//Dye the canvas white
imagecolortransparent($im, $white);//Set the white color in the image to transparent 

imagepng($im, $img_name);//Generate the image 

return true;