var ajax;
var mc;

function replaceMainContent(pageString) {
	mc = document.getElementById("content-container");
	
	var url = "project-pages/page-" + pageString + ".php";
	
	ajax = new XMLHttpRequest(); //new!
	ajax.onreadystatechange = function() { //anonymous function!
		if(ajax.readyState == 4) { //load that shit!
			mc.innerHTML = ajax.responseText; //replace
		}
	}
	ajax.open("GET", url, true);
	ajax.send();
}