innerHTML in PHP

I really hope you guys can help me on this.
To put it simple: in JavaScript there is a simple way to put the HTML inside a div (assuming the id of the div is "content") into a variable.
var string = document.getElementById("content").innerHTML;
Now I want to do the same thing with PHP.
$string = ???
I tried google, but it didn't give me anything of use.
Does anyone know a solution for this?

In case you're not trolling, PHP is run server-side. Which means that it doesn't even know what the "content" element is.

Ha! Right...
I guess I didn't think of that because I hardly use PHP for the things it's meant to do.
Anyways, I'll find a solution.

damn, forgot to sage there.
Anyways, the solution seems obvious now.
Just put the innerHTML via JS into a form input field and then use get method or something.

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