How to pass array back to PHP using Ajax

Question

I have an array but can't complete the final "success" step. I want to populate an html page with array data retrieved from a MySQL query triggered by POST data sent via JQuery Ajax. Console.log shows the array content being returned, but I don't know how to get it for the final step (populate HTML page).

Normally, I use PHP to generate an array and include the HTML page I want to display it on. I like the speed of Ajax, and that the data can be passed without a submit button, but I am stuck. I hope there's enough info below to identify my error(s).

Thanks for any assistance.

Here's my Ajax:

// Get value of drop-down selection
$( "#agent_preview" ).change(function(){
    var broker_agent_id = $( this ).val();
    console.log(broker_agent_id);

    $.ajax({
        url: '../ajax/display-agent-profile.php',
        type: 'POST',            
        data: { broker_agent_id: broker_agent_id },
        cache: false,
        success: function(data){
            $("#preview").html(data);   
        }
    });           
});

Here's the PHP page (display-agent-profile.html.php) to process the Query:

<?php

if(isset($_POST['broker_agent_id'])){

    // Receive value of variable from ajax script @script.js and store in variable
    $broker_agent_id = htmlspecialchars($_POST['broker_agent_id']);

    // Connect to database
    include '../brokers/includes/dbconnect.php';


    // Get broker_id of agent using broker_agents.id
    try {
        $sql = "SELECT broker_id FROM broker_agents
                WHERE id = :id";
        $s = $db->prepare($sql);
        $s->bindValue(':id', $broker_agent_id);
        $s->execute();

        // Store query results in array
        $result = $s->fetch(PDO::FETCH_ASSOC);

        // Store broker id in variable
        $broker_id = $result['broker_id'];               
    } 
    catch (PDOException $e) {
        $errMsg = "Error fetching broker id from database " . $e->getMessage();
        include 'includes/error.html.php';
        exit();
    }

    // Retrieve data for agent based on broker_agents.broker_id and brokers.id (brokers.id = broker_agents.broker_id)
    try {
        $sql = "SELECT  brokers.id, broker_agents.profile_photo, broker_agents.first_name, broker_agents.last_name, brokers.company_logo, brokers.company_name, brokers.address1, brokers.address2, 
                        brokers.telephone, broker_agents.cell, broker_agents.agent_email, brokers.website, brokers.company_bio, broker_agents.about_me, brokers.services, broker_agents.affiliations, 
                        broker_agents.states_served, broker_agents.counties_served
                FROM    brokers
                INNER JOIN broker_agents
                ON      brokers.id = broker_agents.broker_id
                WHERE   broker_agents.id = :id
                AND     broker_agents.broker_id = :broker_id";
        $s = $db->prepare($sql);
        $s->bindValue(':id', $broker_agent_id);
        $s->bindValue(':broker_id', $broker_id);
        $s->execute();

        // Store agent results in associative array
        $agent = $s->fetch(PDO::FETCH_ASSOC); 
    } 
    catch (PDOException $e) {
        $errMsg = "Error fetching agent data from database " . $e->getMessage();
        include 'includes/error.html.php';
        exit();
    }

    // Disconnect 
    $db = NULL;       

    include 'profile-preview.html.php';
    exit();
}
?>