title: imagecolorallocatealpha returns white image
body: <p>I have a problem with the imagecolorallocatealpha in PHP. When setting the opacity to 127, I get a white image instead of a transparent one.</p>
  
  <p>Here is my code</p>
  
  <pre><code>$image = imagecreatetruecolor($width, $height);
  imagesavealpha($image, true);
  $color = imagecolorallocatealpha($image, 255, 255, 255, 127);
  imagefill($image, 0, 0, $color);
  header('Content-type: image/png');
  imagepng($image);
  imagedestroy($image);
  exit;
  </code></pre>
  
  <p>I have also tried this one but I get the same result</p>
  
  <pre><code>$image = imagecreatetruecolor($width, $height);
  $x = imagecolorat($image, 0,0);
  imagecolortransparent($image, $x);
  header('Content-type: image/png');
  imagepng($image);
  imagedestroy($image);
  exit;
  </code></pre>
  
  <p>Any idea ? Can it be related to server configuration ?</p>