How to set a transparent background with GD in PHP? -

we have script generates mp3 waveform images, , uses specified foreground , background color. i'd have background transparent.

i've messed below script bit have no idea i'm doing. ideas on how generates foreground color , not background?

 /**        * image generation        */       // how detail want. larger number means less detail       // (basically, how many bytes/frames skip processing)       // lower number means longer processing time       define("detail", 5);        // user vars 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 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 , 255         $v = (int) ($data[$d] / 255 * $height);         // draw line on image using $v value , centering vertically on canvas         imageline($img, $d / detail, 0 + ($height - $v), $d / detail, $height - ($height - $v), imagecolorallocate($img, $r, $g, $b));       }        $outpngname= $outputname.".png"; 

this looking for.

    // fill background transparent color / white     $transcolor = imagecolortransparent($img, imagecolorallocatealpha($img, 255, 255, 255, 127));     imagefill($img, 0, 0, $transcolor);