Get changes of the value of a variable in php in JQuery

I want to pass a value that is given in a PHP variable $_SESSION['T']to JQuery . This variable changes its value as the "button" type inputs that I have in HTML5 are pressed . For the "Previous" input, the variable decreases (-) and of course, for the "Next" input , the variable increases (+). This is perfect.

Now what I want is that when it reaches the first record (it is achieved by clicking on the "Previous" input ) that same button is disabled and if it reaches the end (click on the "Next" input ) the same.

The problem is that I cannot achieve this from JQuery , although I can see the value from the console (for example, register 21, which is the one it starts with) when I click on the "Previous" or "Next" buttons that variable T that is in PHP (in JQuery it is named as Nro ) does not change. Therefore it seems that it is always in the same record (21), when it should change to number 20 (for "Previous" ) or 22 (for "Next" ).

This is a part of the code I have from PHP :

session_start();
if (!isset($_SESSION['T'])) {
    $_SESSION['T']=21;
}else{
    if(isset($_POST['Ant']) && !empty($_POST['Ant'])) {
         $_SESSION['T']--;
    }
    if(isset($_POST['Sig']) && !empty($_POST['Sig'])) {
         $_SESSION['T']++;
    }
}
   $T=$_SESSION['T'];

And the code I have for both buttons from JQuery is:

$('body').on('click', '#Ant, #Sig', function(e) {
    e.preventDefault();
    var $btn = $(e.target);
    $(".Tips").hide();
    $("#carga").show();
    var dataString = $('#FrmBus').serialize() + '&' + 
    encodeURI($btn.attr('name')) + '=' + encodeURI($btn.val());
    $.ajax({
        url: "http://localhost:81/prueba/",type: "POST",
        data:dataString,
        cache:false,
        success:function(data)
        {
             $("#carga").hide();
             $(".Tips").show();
             var $response=$.parseHTML(data);
             $response=$('.Tips',$response).html();
             $(".Tips").html($response);
             $("input:button").button();
            //$btn.prop('disabled', false);
            var Nro = <?php echo $T?>;
            if (Nro==1){
                $("#Ant").prop('disabled',true);
            }else if (Nro==1){
                $("#Sig").prop('disabled',true);
            }else{
                console.log("El registro actual es el "+ Nro);
            }
        }});
       return false;
});

Perhaps it cannot be manipulated in that way, I am talking about the line var Nro = <?php echo $T?>;I have also tried with json_encode ()and it does not work for me either. Is it a server side settings issue?