Missing span style tag from ajax jQuery to PHP

Hi, im trying to implements CKeditor on Codeigniter framework. I can load, instance and config as well but when i submit textarea's content with jQuery ajax in JSON string format, span's style tag disappear on PHP server side.

Here is my javascript function

function debug(){
    var vcadena = JSON.stringify($("#ncuerpo").val(), null, 2);
    $.ajax({
        type: "POST",
        url: "/administrar/noticias/serverdebug",
        data: ({cadena:vcadena})
    });
}

Everything is ok until data send:

public function serverdebug(){
       $datos = json_decode($this->input->post('cadena'));
        echo $datos;
    }

I'm sure than vcadena var is OK on javascript with this data:

<p><span style="color:#CCCCCC">Im a colored string</span></p>

But when i try to print posted data on PHP server side style tag is missing:

<p><span>Im a colored string</span></p>

I'm really frustrated, whats wrong ? I was looking for hours and nothing helps me.
Regards