(バージョン情報なし。おそらく SVN 版にしか存在しないでしょう)
Imagick::__toString — Returns the image as a string.
Returns the current image as string. This will only return a single image; it should not be used for Imagick objects that contain multiple images e.g. an animated GIF or PDF with multiple pages.
この関数にはパラメータはありません。
Returns the string content on success or an empty string on failure.