/**
 *
 * 判断一个图片是否是包含alpha通道的png
 * @param string $file
 */
function is_alpha_png($file)
{
    if (!file_exists($file)) {
        return false;
    }
    $f = @fopen($file, 'r');
    if (!$f) {
        return false;
    }
    $bin = fread($f, 29);
    fclose($f);
    $info = @unpack("C8c/C8char/C4width/C4height/Cdepth/Ccolortype", $bin);
    $png = array(
        137,
        80,
        78,
        71,
        13,
        10,
        26,
        10
    );
    //判断头是否是png文件
    for ($i = 0; $i < 8; $i++) {
        if ($png [$i] != $info ['c' . ($i + 1)]) {
            return false;
        }
    }
    list ($width, $height) = getimagesize($file);
    //这里用width3和4就可以了
    $w = $info ['width3'] * 256 + $info ['width4'];
    $h = $info ['height3'] * 256 + $info ['height4'];
    //判断当前获取跟系统获取的值是否相同
    if ($width != $w || $height != $h) {
        return false;
    }
    $depth = $info ['depth'];
    $colorType = $info ['colortype'];
    if ($depth == 8 || $depth == 16) {
        if ($colorType == 6) {
            return true;
        }
    }
    return false;
}