You don't develop mobile apps in PHP. You develop mobile apps in HTML, CSS and JavaScript. - At least in the context of web-based mobile apps. Native apps are a whole other thing, requiring device specific tools.
PHP is the back-end providing data and server-side logic for the web-app. It doesn't care what kind of devices the front-end is targeting.
No, not really. These are just normal websites set up to look their best on small screens, perhaps using media-queries to show up differently on different screen sizes or device types.
There are also things like jQuery Mobile that can be used for mobile development, but that's of very little use without proper knowledge of HTML, CSS and JavaScript first.
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