Programming a website in PHP will give you an extensive amount of pre-written resourses either Open Source or Paid. It has the most out of all the laugages. With PHP and MySQL combined you can store information in to a database and Display, Add, Edit Content plus much more.
Back in the old days one would have to create different sites to accommodate this same responsive function. Imagine having to create a mini site for all the types of phones or tablets back them.
CSS is designed primarily to enable the separation of document content from document presentation, including aspects such as the layout, colors, and fonts. This separation can improve content accessibility, provide more flexibility and control in the specification of presentation characteristics, enable multiple PHP pages to share formatting by specifying the relevant CSS in a separate .css file, and reduce complexity and repetition in the structural content.
This separation of formatting and content makes it possible to present the same markup page in different styles for different rendering methods, such as on-screen, in print, by voice (when read out by a speech-based browser or screen reader) and on Braille-based, tactile devices. It can also be used to display the web page differently depending on the screen size or device on which it is being viewed. Readers can also specify a different style sheet, such as a CSS file stored on their own computer, to override the one the author has specified.
Changes to the graphic design of a document (or hundreds of documents) can be applied quickly and easily, by editing a few lines in the CSS file they use, rather than by changing markup in the documents.
PRO’s OF A RESPONSIVE PHP AND CSS SITE:
The list is quite vast, but in short whats nice about it is there is a considerable amount of pre-written code ( Why re-invent the wheel? ) which makes it faster for a designer to create a website.
At this time there really are not any con's to mention, except it is not going to be W3C.