Using Javascript / Ajax to run a php script

I have a button at the bottom of my form, I am trying to disable it if the email that the user tries to use is already in use. The email the user supplies is $email

Here is my front-end code..

        <script type="text/JavaScript">
        var email = '<?php echo $email; ?>';//Get the value in the username textbox
        $.ajax({  //Make the Ajax Request
            type: "POST",  
            url: "ajax_check_email.php",  //file name
            data: "email="+ email,  //data
            success: function(server_response){  

           $("#availability_status").ajaxComplete(function(event, request){ 

            if(server_response == '0')//if ajax_check_username.php return value "0"
            { 
             alert("Server response recieved: 0");
            $("#availability_status").html('<img src="/img/icon-success.png" align="absmiddle" style="height: 15px; width: auto; margin-top: -2px;"> <font color="#c9dc54" style="padding-top: 5px;"> Available </font>  ');
            $("#btn-submit-2").html('<button type="submit"  class="btn btn-danger btn-lg btn-block"   id="b"  style="font-family: "klavikaRegular"; letter-spacing: 1px;" name="skip" value="0">CONTINUE</button>');
            //add this image to the span with id "availability_status"
            }  
            else if(server_response == '1')//if it returns "1"
            {  
             alert("Server response recieved: 1");
             $("#availability_status").html('<img src="/img/icon-error.png" align="absmiddle" style="height: 15px; width: auto; margin-top: -2px;"> <font color="#b11116" style="padding-top: 5px;">Not Available </font>');
             $("#btn-submit-2").html('<a class="btn btn-default btn-lg btn-block" style="font-family: "klavikaRegular"; letter-spacing: 1px;">CONTINUE</a>');
            }  

           });
           } 

          }); 

        }
        else
        {
        alert("failed query");
        $("#availability_status").html('<font color="#b11116">Username too short</font>');
        $("#btn-submit").html('<a class="btn btn-default btn-lg btn-block" style="font-family: "klavikaRegular"; letter-spacing: 1px;">USERNAME TOO SHORT</a>');
        //if in case the username is less than or equal 3 characters only 
        }

        </script>

Here is my php file source

    <?php

    include('database_connection.php');
    //Include The Database Connection File 
    if(isset($_POST['email']))//If a username has been submitted 
    {
    $email = mysql_real_escape_string($_POST['email']);//Some clean up :)

    $check_for_email = mysql_query("SELECT user_login FROM wp_users WHERE user_email='$email'");
    //Query to check if username is available or not 

    if(mysql_num_rows($check_for_email))
    {
    echo '1';//If there is a  record match in the Database - Not Available
    }
    else
    {
    echo '0';//No Record Found - Username is available 
    }

    }

    ?>

Is there an issue with my javascript syntax?

$email is hard coded as something I know should fail..