如何设置一个透明的背景与GD在PHP

我们有一个生成MP3波形图像的脚本，它目前使用指定的前景和背景颜色。我想让背景完全透明。

我已经打乱了下面的脚本一点，但我不知道我在做什么。关于如何让它只生成前景色和不背景有什么想法吗?

 /**
       * Image generation
       */
      // how much detail we want. Larger number means less detail
      // (basically, how many bytes/frames to skip processing)
      // the lower the number means longer processing time
      define("DETAIL", 5);
      // get user vars from form
      $width = isset($_POST["width"]) ? (int) $_POST["width"] : 775;
      $height = isset($_POST["height"]) ? (int) $_POST["height"] : 122;
      $background = isset($_POST["background"]) ? $_POST["background"] : $background_color;
      $foreground = isset($_POST["foreground"]) ? $_POST["foreground"] : $foreground_color;
      // create original image width based on amount of detail
      $img = imagecreatetruecolor(sizeof($data) / DETAIL, $height);
      // fill background of image
      list($r, $g, $b) = html2rgb($background);
      imagefilledrectangle($img, 0, 0, sizeof($data) / DETAIL, $height, imagecolorallocate($img, $r, $g, $b));
      // generate background color
      list($r, $g, $b) = html2rgb($foreground);
      // loop through frames/bytes of wav data as genearted above
      for($d = 0; $d < sizeof($data); $d += DETAIL) {
        // relative value based on height of image being generated
        // data values can range between 0 and 255
        $v = (int) ($data[$d] / 255 * $height);
        // draw the line on the image using the $v value and centering it vertically on the canvas
        imageline($img, $d / DETAIL, 0 + ($height - $v), $d / DETAIL, $height - ($height - $v), imagecolorallocate($img, $r, $g, $b));
      }
      $outPngName= $outputName.".png";