$(function() {
$('nav a').click(function(e) {
 href = $(this).attr("href");

 loadContent(href); //function to call on nav click event

 history.pushState("", "", href); //append new href to end of url
 e.preventDefault();
});

window.onpopstate = function(event) { //on a url change (back / forward browser button)
 loadContent(location.pathname); //load content for that specific url
};
});

function loadContent(url){
$.ajax({
 type: "GET",
 url: "content_manager.php",  //make ajax request to this php file
 data: {"content_href" : url}, //send it the href
 success: function(data) {
  $("#content").html(data); //get returned data put it into the #content div
 }
});
 
$('li').removeClass('current'); //make it pretty
$('a[href="'+url+'"]').parent().addClass('current'); //make it pretty
}

<?php

$content_href = $_GET["content_href"]; //get the content href sent by the ajax call

switch(content_href){ //switch
      //different cases to test for (yes I'm aware they do different things this is the bit where it's messing up so I'm experimenting)
case "portfolio": echo("<p>include/portfolio.php</p>"); break;
case "about": echo("<p>include/about.php"</p>); break;
case "resume": $content_href = "include/resume.php"; break;
case "contact": $content_href = "include/contact.php"; break;
default: echo("<p>include/404.php</p>"); //this one works but the others don't..
}
?>

<?php

$content_href = $_GET["content_href"];

switch($content_href){
case "portfolio": include("include/portfolio.php"); break;
case "about": include("include/about.php"); break;
case "resume": include("include/resume.php"); break;
case "contact": include("include/contact.php"); break;
default: include("include/404.php");
}
?>