php画一个背景透明，且开始处旋转到在圆的正下方处的一个圆饼统计图

/* * _survey 得到一个投票的圆饼图* @access public 表示函数对外公开* @param $_agree 同意票数* @param $_disagree 反对票数* return 一张统计圆饼图* */function _vote($_agree,$_disagree){    //处理参数    if($_agree==0&&$_disagree==0){        $_vote = 270;    }    if($_agree==0&&$_disagree!=0){        $_vote = 91;    }    if($_agree!=0&&$_disagree==0){        $_vote = 89;    }    if($_agree!=0&&$_disagree!=0){        $_vote = intval(360*($_agree/($_agree+$_disagree))+90);    }    //创建画布    $_img=imagecreatetruecolor(150,100);    //填充透明背景    imagealphablending($_img, false);    imagesavealpha($_img, true);    $white = imagecolorallocatealpha($_img,255,255,255,127);    imagefill($_img,0,0,$white);    //创建颜色    $_green=imagecolorallocate($_img,180,210,52);    $_blue=imagecolorallocate($_img,83,201,237);    $_gary=imagecolorallocate($_img,192,192,192);    $_green1=imagecolorallocate($_img,149,178,36);    $_blue1=imagecolorallocate($_img,41,160,193);    $_gary1=imagecolorallocate($_img,144,144,144);        for($i=60;$i>=50;$i--){        imagefilledarc($_img,100,$i,100,50,90,$_vote,$_blue1,IMG_ARC_PIE);        imagefilledarc($_img,100,$i,100,50,$_vote,90,$_green1,IMG_ARC_PIE);     }        imagefilledarc($_img,100,$i,100,50,90,$_vote,$_blue,IMG_ARC_PIE);    imagefilledarc($_img,100,$i,100,50,$_vote,90,$_green,IMG_ARC_PIE);        header("content-type: image/png");    imagepng($_img);    imagedestroy($_img);}