To make a mobile "app", you are going to have to learn something besides PHP. Remember that PHP is a server technology. So while you may use PHP for the server component, the UI is built around the client.
The issue you'll encounter with mobile apps is that the client language will vary from platform to platform. You may actually end up writing 4 mobile apps. The good news is that there are a few utilities out there to convert an app from one language to another. With that in mind, your best bet will be to select a platform to start from and a language or SDK that is most familiar to you.
iPhone and Android are both well-documented online and have extensive SDKs, so you may want to start there.
Now if you just want to develop a mobile version of a web page that will be cross-platform compatible, there are some things you can do with Ajax to make the web page app-like, such as buttons that do live database updates without reloading the page.