PHP没有收到$ _POST

I have a problem with my PHP, I'm doing an ajax post with jquery. The problem: my PHP doesnt receive any POST, with GET there are no problems. I'm using the last jquery version. Ajax doesnt throw any error, I tried with error:function(...) and console.log.

idCat is a number, for example: 3 $nuevaFila and other undeclared variables are previously declared, they aren't constructive.

PHP returns well the HTML because I try any html and I receive it ok

The ajax code is this one:

$.ajax({
    cache:false,
    type:'post',
    url:'admin/categoria_ajax_ad',
    data: {id: idCat},
    success: function(htmlFila) {       
        var nFila=$cFilas.find(".fila").size()+1;
        $nuevaFila.html(htmlFila);
        $nuevaFila.appendTo($cFilas);
    },
});

Try using dataType (xml, json, script, or html) setting parameter like this depending on your return data type -

$.ajax({
cache:false,
type:'post',
url:'admin/categoria_ajax_ad',
data: {id: idCat},
dataType : "string"
success: function(htmlFila) {       
    var nFila=$cFilas.find(".fila").size()+1;
    $nuevaFila.html(htmlFila);
    $nuevaFila.appendTo($cFilas);
    },
});