creating new class from wrapper class (OOP question)

In a image gallery I have to class'es: class GPicFilePicture extends GPicFileType class GPicFileMovie extends GPicFileType both of them are based on: abstract class GPicFileType In my code I need to create a new GPicFilePicture. To avoid duplicated code I've create a wrapper class: class GPicFile that does something like this: if (fileextension == "jpg") return new GPicFilePicture(); else return new GPicFileMovie(); so my PHP code looks like: $file = new GPicFile($filename); getFileDate() is implemented in both GPicFilePicture and GPicFileMovie. Now I try to do: $file->getFileDate(); I get an error saying: Call to undefined method GPicFile::getFileDate() Where am I going wrong?