What code do I use to share the content of a page with a photo, title and specific description on facebook?

In fact, this is neither php nor css, but html!

<meta property="og:title" content="Título" />
<meta property="og:description" content="Descrição" />
<meta property="og:image" content="Link_da_Imagem" />

Just put this in your head, and change the content attribute of each tag. The first is the title, the second the description and the third is the link of the image.

In this case, you will add this to the page that will be shared, sometimes it takes some time for Facebook to notice!

PS: I suggest you do a more thorough search next time, if you put your question on Google, you'll find what you need.