PHP 生成等比居中水印图

/**
 * 图片增加水印
 * @param $img 待加水印的图片地址
 * @param $watermark 水印图片的地址
 * @param int $district
 * @param int $watermarkquality
 * @return bool|void
 */
function watermark($img, $watermark, $district = 5,$watermarkquality = 95){
    $imginfo = getimagesize($img);

    $width = $imginfo[0];
    if($imginfo[0] > $imginfo[1]) $width = $imginfo[1];
    $width = $width / 2;

    $size = 'static/layuimini/images/thum'.time().'.png';
    if(resize_image($watermark,$size,$width ,$width)) $watermark = $size;

    $watermarkinfo = getimagesize($watermark);

    $img_w = $imginfo[0];
    $img_h = $imginfo[1];

    $watermark_w = $watermarkinfo[0];
    $watermark_h = $watermarkinfo[1];

//    $watermark_w = $imginfo[0] / 2;
//    $watermark_h = $imginfo[1] / 2;

    if($district == 0) $district = rand(1,9);

    if(!is_int($district) OR 1 > $district OR $dis