PHP裁剪图像的底部

Question

所以我有这样的图像 - ＆gt; http://i.imgur.com/bHdcBhd.png我需要裁掉红色区域。我尝试裁剪它，但功能是从顶部和底部裁剪整个图像。那么如何只裁剪图像的底部呢？最终结果是错误的（http://i.imgur.com/semxwyY.png）以下是代码：

function getWidth() {
    return imagesx($this->image);
}

function getHeight() {
    return imagesy($this->image);
}

function cutTheBottom($width, $height){
    $height = $this->getHeight() - 55;
    $width = $this->getWidth();

    $x = ($this->getWidth() / 2) - ($width / 2);
    $y = ($this->getHeight() / 2) - ($height / 2);

    return $this->cut($x, $y, $width, $height);
}

function cut($x, $y, $width, $height)
{
    $new_image = imagecreatetruecolor($width, $height);

    imagecolortransparent($new_image, imagecolorallocate($new_image, 0, 0, 0));
    imagealphablending($new_image, false);
    imagesavealpha($new_image, true);

    imagecopy($new_image, $this->image, 0, 0, $x, $y, $width, $height);

    $this->image = $new_image;
}