if(isset($_COOKIE['fid'])):
   // do something
endif;

$('#login').click(function(){
     _gaq.push(['_trackEvent', 'viewerChose', $.cookie('whichSide')+'side']);
});

$("#login-btn").click(function(){
    $("#login-load-icon").css("display") = "block";
    $.post( "example.php", { user: "example", pass: "example").done(function(data){
        if(data == 1){
            location.reload();
        } else {
            $("#login-error").css("display") = "block";
            $("#login-load-icon").css("display") = "none";
        }
    });
});

if(isset($_POST['user']) && isset($_POST['pass'])):
    if(strtoupper($_POST['user']) == strtoupper("This would a Database Query")):
        if($_POST['pass'] == "This would you matching the hashed pass with the hash in the DB"):
            setcookie("fid", "LOGIN KEY HERE", time()+3600, "/");
        endif;
    endif;
endif;
// etc...

if(isset($_COOKIE['fid')):
        if($_COOKIE['fid'] == $Database['login_key']):
            // get all the user details and they're logged in
        else:
            // they tried to trick your system
    else:
        // load the login-btn
    endif;