Trying to retrieve username from the database jquery
echo "<form method='post' action='regprocess.php' id='registerform'>";
        echo '<fieldset class="register">';
        echo"<h2>Register</h2>";
            echo "<ul>";
                    echo '<li><label for="FirstName">First Name: </label> <input type="text" name="FirstName" id="FirstName"></li>';
                    echo '<li><label for="LastName">Last Name: </label> <input type="text" name="LastName" id="LastName"></li>';
                    echo '<li><label for="Email">Email: </label><input type="email" name="Email" id="Email"></li>';
                    echo '<li><label for="Username">Username: </label><input type="text" name="Username" id="Username"></li>';
                    echo '<li><input type="button" id="check_username_availability" value="Check Availability"></li>';
                    echo '<div id="username_availability_result"></div>';
                    echo '<li><label for="Password">Password: </label><input type="password" name="Password" id="Password"></li>';
                    echo '<li><input type="submit" value="Register"></li>';
                    echo "</ul>";
        echo "</fieldset>";
        echo "</form>";


<script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.2/jquery.min.js"></script>

<SCRIPT LANGUAGE="JAVASCRIPT" TYPE="TEXT/JAVASCRIPT">
$(document).ready(function() {

        var checking_html = 'Checking...';

        //when button is clicked
        $('#check_username_availability').click(function(){
            //run the character number check
                $('#username_availability_result').html(checking_html);
                check_availability();
        });

  });

//function to check username availability
function check_availability(){

        //get the username
        var username = $('#username').val();

        //use ajax to run the check
        $.post("check_username.php", { username: username },
            function(result){
                //if the result is 1
                if(result == 1){
                    //show that the username is available
                    $('#username_availability_result').html(username + ' is Available');
                }else{
                    //show that the username is NOT available
                    $('#username_availability_result').html(username + ' is not Available');
                }
        });

}
</script>


<?php
$conn = new mysqli('sapphire', 'cgreenheld', '', 'cgreenheld_dev');
$username = mysqli_real_escape_string($conn, $_POST['Username']);

//mysql query to select field username if it's equal to the username that we check '
$usernameresult = 'Select Username from User where Username = "'. $username .'"';
$uresult = $conn->query($usernameresult);

//if number of rows fields is bigger them 0 that means it's NOT available '
if($uresult->num_rows==1) {
    //and we send 0 to the ajax request
    echo 0;
}else{
    //else if it's not bigger then 0, then it's available '
    //and we send 1 to the ajax request
    echo 1;
}
?>

echo '<li><label for="Username">Username: </label><input type="text" name="Username" id="Username"></li>';

var username = $('#username').val();

$.post("check_username.php", { username: username },

$username = mysqli_real_escape_string($conn, $_POST['Username']);