Request ajax does not return what I want

I have a registration page with a form, the person types the year and clicks the button to search the register. With the typed year I have to make the request for PHP to execute the query, return the data and distribute the data to the fields of the form.

I'm trying to make a simple request. The page takes the base year of the form and sends a request with the base year returning the result of the query within PHP. I have no experience with HTML, PHP or javascript and after weeks studying how to do this I got here. The request executes and returns, but the result is not what I want. Using $ .post the result that exits in alert (I'm printing the result to see if it is returning what I want, I'll handle the query data by distributing them to each form field) is the PHP code of the query. php instead of executing and returning the query. I tried using $ .ajax and the result was even more bizarre because it not only executes query.php, but returns the HTML code of the page itself. I tried doing using XMLHttpRequest () and also returns the code instead of executing. I do not know what I'm doing wrong. Help ...

Edited: I built the code with $_POST and tag <?php and the request worked. The problem now is simply to handle the data. I do not know how to return or how to treat. In short: I have the request that makes the query in the DB and returns to the page. The request does not return (or does not execute, I do not know). alert of $.post does not execute when I use json_encode . If I return text it only returns array (I saw the variable by F12 and the alert printa that too). What I need is that at the end of it all I have an array (or anything easy to separate) to assign each column value queried in the DB to the specific HTML field.

Using $ .post:

$.post("query.php", {ano_ref: ano_base},
      function(retorno){
    alert(retorno);
}, "json");

With $ .ajax it looks like this:

$.ajax({
    type: "POST",
    URL: "query.php",
    datatype: "json",
    data:{ano_ref: ano_base},
    success: function (resposta){
        alert(resposta);            },
    error: function(){
        alert("erro de req.");
    }       
})

Using XMLHttprequest:

var xhttp;
if (window.XMLHttpRequest) {
    xhttp = new XMLHttpRequest();
    } else {
    // code for IE6, IE5
    xhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
xhttp.open("POST", "query.php", true);
xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
        alert(this.responseText);
    }
};

xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
var string = "ano_ref=" + encodeURIComponent(ano_base.toString());
alert(string);
xhttp.send(string);

The query.php:

<?php

    include("conexao_postgres.php");

    $ano_base = $_POST["ano_ref"];

    $sql = "SELECT * FROM public.parametros_estaduais WHERE ano_base = $ano_base";
    $resultado = pg_query($dbconn, $sql);
    $resultado_array = pg_fetch_row($resultado, 0); // ($resultado)

    echo json_encode($resultado);
?>

Enjoying, can I return a single array or just json same? Is json too hard to handle? Like I said, each of these query columns goes to a form field, that's all.