Fetching row from database using ajax and php

I have been trying to echo users profile photo using ajax and php but i get nothing but a blank page. Below is my ajax and php code which I have written myself.

Ajax

$('#profile_photo').html('<img src="loading.gif" alt="loading..." width="20px" height="20px" >');

$(document).ready(function() {
    $.ajax( {    
        type: "GET",
        url: "profile_photo.php",             
        dataType: "html",   
        success: function(d) {                    
             $("#profile_photo").html(d); 
        }
    });
});

PHP (profile_photo.php)

<?php  
    // connect to db
    include 'db.php';

    // start session
    session_start();

    // user id 
    $id = $_SESSION['id'];
    $user = $_SESSION['name'];

    // variables

    // profile photo
    $profile_photo_query = mysqli_query($db_var, "SELECT * FROM users_profile_photo WHERE id = '$id'"); 
    while ($row = mysqli_fetch_array($profile_photo_query, MYSQLI_ASSOC)) {
        $set_profile_photo = $row["image"];
    }

        // for profile photo (encode with base64)
        if ($set_profile_photo == null) {
            echo "<img src='data:image/png;base64,".base64_encode(file_get_contents("profile_photo/default.png"))."' alt='".$user."' title='".$user."' onContextMenu='return false;'";
        } else {
            echo "<img src='data:image/png;base64,".base64_encode(file_get_contents("profile_photo/$set_profile_photo"))."' alt='".$user."' title='".$user."' onContextMenu='return false;'";
        }
?>

Html

<div class="prev">
   <span id="profile_photo"></span>
</div>

Css (prev)

.prev {
    width: 120px;
    height: 120px;
    border: 5px solid #fff;
    border-radius: 50%;
    -webkit-border-radius: 50%;
    -moz-border-radius: 50%;
    overflow: hidden;
    margin-left: 10px;
    background-color: white;
    position: absolute;
    margin-top: -80px;
    z-index: 1000;
}
.prev img {
    width: 100%;
    height: 100%;
}

The ajax loads the loading image (.gif) successfully but shows a blank page when loading image is done loading. I will like to know why the profile photo is not been shown because my php code has no errors.