Code:

  public function paddedImage ($src_filename, $dest_filename, $_width, $_height, $bg_R, $bg_G, $bg_B, $quality) {
    
    if(!is_dir($src_filename))
    {    
      $ext = strtolower(pathinfo($src_filename, PATHINFO_EXTENSION));
      
      switch(strtolower($ext))
      {
        case "jpg":
        case "jpeg":
          $src = imagecreatefromjpeg($src_filename);
          break;
        case "png":
          $src = imagecreatefrompng($src_filename);
          break;
        case "gif":
          $src = imagecreatefromgif($src_filename);
          break;
      }
      
      $width = @imagesx($src);
      $height = @imagesy($src);
      
      if($width != 0 && $height != 0)
      {
        $ratio_w = $_width / $width;
        $ratio_h = $_height / $height;

        $ratio = ($ratio_w < $ratio_h) ? $ratio_w : $ratio_h;

        $new_width = $width * $ratio;
        $new_height = $height * $ratio;

        $padd_x = ($_width - $new_width) / 2;
        $padd_y = ($_height - $new_height) / 2;

        $dest = imagecreatetruecolor($_width, $_height);
        $background_color = imagecolorallocate($dest, $bg_R, $bg_G, $bg_B);
        imagefilledrectangle($dest, 0, 0, $_width, $_height, $background_color);

        imagecopyresampled($dest, $src, $padd_x, $padd_y, 0, 0, $new_width, $new_height, $width, $height);

        if ($dest_filename) {
            imagejpeg($dest, $dest_filename, $quality);
        } else {
            header('Content-Type: image/jpeg');
            imagejpeg($dest, null, $quality);
        }

        imagedestroy($dest);
        imagedestroy($src);
      }
    }
  }

Code:
paddedImage($src_filename, $dest_filename, 313, 235, 255, 255, 255, 100);