Using jQuery you can do this:
HTML:
<ul class="nav">
<li><a href="javascript:void(0);" name="section-filename-here">Item</a></li>
<li><a href="javascript:void(0);" name="section-filename-here">Item</a></li>
<li><a href="javascript:void(0);" name="section-filename-here">Item</a></li>
<li><a href="javascript:void(0);" name="section-filename-here">Item</a></li>
<li><a href="javascript:void(0);" name="section-filename-here">Item</a></li>
<li><a href="javascript:void(0);" name="section-filename-here">Item</a></li>
</ul>
<div id="content"></div>
Where you see name="section-filename-here", only input the name of the file without the .php extension as this can be added in the jQuery as in the example below
Javascript (jQuery/aJax):
$('ul.nav li a').click(function () {
var thisSection = $(this).attr('name');
$.ajax({
url: "/path-to-sections/"+thisSection+".php",
cache: false,
success: function (data) {
$('#content').html(data);
},
error: function () {
// only put something here if you want to show an error
}
});
});