Problem loading data using PDO: encoding error - php

Question

I want to load some data from sql table into my page. This is the php script that I am using:
PHP:
<?php
include_once "conectar.php";
header('Content-Type: text/txt; charset=utf-8');
public class Calendario{
public function cargarCalendarios() {
$conexion = new PDO("mysql:host=localhost;dbname=practica_db;charset=utf8","root","");
$sql = "SELECT nombre, descripcion, likes FROM calendarios";
$pdo = $conexion->prepare($sql);
$pdo->execute();
$json = array();
while ($row=$pdo->fetch(PDO::FETCH_ASSOC)) {
$json[] = array('nombre' =>$row['nombre'], 'descripcion'=>$row['descripcion'], 'likes'=>$row['likes']);
}
echo json_encode($json);
}
}
($calendario = new Calendario())->cargarCalendarios();
?>
I am calling that script here:
JS:
function cargarCalendarios() {
$.ajax({
url: '../php/menuCalendarios.php',
type: 'GET',
dataType: 'json',
})
.done(function(data) {
$.each(data, function(id,value){
//code
}
});
})
.fail( function( jqXHR, textStatus, errorThrown ) {
//...More error handling
} else if (textStatus === 'parsererror') {
alert('Requested JSON parse failed.');
});
}
I get an error in the request, and I don't know what I am doing wrong. The error must be in the PHP code I asume, but I just can't find anything.
If it helps, I am working in a local server with XAMPP.
I always get failure and the alert:
'Requested JSON parse failed.'