Checkbox checked mit AJAX

<div class="modal fade" id="modal_kunde_hinzufuegen" tabindex="-1" role="dialog" aria-labelledby="label_kunde_hinzufuegen">  
  <div class="modal-dialog" role="document">  
    <div class="modal-content">  
      <div class="modal-header">  
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>  
        <h4 class="modal-title" id="label_kunde_hinzufuegen"><?php echo $customers['new']; ?></h4>  
      </div>
	  <div class="modal-body">  
		  <div class="form-group">  
			<label for="vorname"><?php echo $customers['firstname']; ?></label>  
			<input type="text" id="firstname_neu" class="form-control" placeholder="<?php echo $customers['firstname']; ?>">  
		  </div>
		  <div class="form-group">  
			<label for="nachname"><?php echo $customers['lastname']; ?></label>  
			<input type="text" id="lastname_neu" class="form-control" placeholder="<?php echo $customers['lastname']; ?>">  
		  </div>
		  <div class="form-group">  
			<label for="tagid1"><?php echo $customers['tagid1']; ?></label>  
			<input type="text" id="tagid1_neu" class="form-control" placeholder="<?php echo $customers['tagid1']; ?>">  
		  </div>
		  <div class="form-group">  
			<label for="tagid2"><?php echo $customers['tagid2']; ?></label>  
			<input type="text" id="tagid2_neu" class="form-control" placeholder="<?php echo $customers['tagid2']; ?>">  
		  </div>
      <div class="form-check">  
      <input class="form-check-input" type="checkbox" value="0" id="admin_check">  
      <label class="form-check-label" for="admin_check">  
       Admin Rechte
      </label>
      </div>
      <div class="form-group">  
			<label for="username" id="username" ><?php echo $customers['username']; ?></label>  
			<input type="text" id="username_neu" class="form-control" placeholder="<?php echo $customers['username']; ?>">  
		  </div>
      <div class="form-group">  
			<label for="password" id="password" ><?php echo $customers['password']; ?></label>  
			<input type="text" id="password_neu" class="form-control" placeholder="<?php echo $customers['password']; ?>">  
		  </div>
		</div>

<script type="text/javascript">  
	$(document).ready(function() {
	   
     // Username und Passwort Feld ausblenden
     $("#username_neu,#username").hide();   
     $("#password,#password_neu").hide();    
     
     //Username und Passwort Feld einblenden wenn checkbox aktiv
      $('#admin_check').change(function(){  
			$("#username,#username_neu,#password,#password_neu").show();  
			$("#admin_check").prop('disabled', true);  
		});
    
    //Implementierung TableSorter für Tabelle Kunden
    $(function(){
			$("#tbl_kunden").tablesorter();  
		});	
      
   //Action bei klick auf Kunde hinzufügen
		$('#btn_kunde_hinzufuegen').click(function(){  
			var firstname = $("#firstname_neu").val();  
			var lastname = $("#lastname_neu").val();  
			var tagid1 = $("#tagid1_neu").val();  
			var tagid2 = $("#tagid2_neu").val();  
      var username = $("#username_neu").val();  
      var password = $("#password_neu").val();  
      var admin = $("#admin_check").val();  
   
      
			$.ajax({
				url: "/Login-System/pages/ajax.php",  
				type: "POST",  
				data: { ACTION : "kunde_hinzufuegen", FIRSTNAME : firstname, LASTNAME : lastname, TAGID1 : tagid1, TAGID2 : tagid2, USERNAME : username, PASSWORD : password, ADMIN : admin},  
				dataType: "text",  
				success: function(data){
					$('#modal_kunde_hinzufuegen').modal('hide');  
					//window.location = "kunden"; 
				},
				error: function(xhr, status, error){
					alert(error + " Fehler beim Einfuegen des Kunden");  
				}
			});
			
		});

	if($_POST['ACTION'] == "kunde_hinzufuegen")  
	{
   $firstname = $_POST['FIRSTNAME'];  
   $lastname = $_POST['LASTNAME'];  
   $tagid1 = $_POST['TAGID1'];  
   $tagid2 = $_POST['TAGID2'];  
   $username = $_POST['USERNAME'];  
   $password = $_POST['PASSWORD'];  
   $admin = $_POST['ADMIN'];  
		
		$sql = "INSERT INTO customers(firstname, lastname, tagid1, tagid2, username, password, admin) VALUES(:firstname, :lastname, :tagid1, :tagid2, :username, :password, :admin)";  
		$stmt = $dbh->prepare($sql);
		$stmt->bindParam(':firstname', $firstname);  
		$stmt->bindParam(':lastname', $lastname);  
		$stmt->bindParam(':tagid1', $tagid1);  
		$stmt->bindParam(':tagid2', $tagid2);  
    $stmt->bindParam(':username', $username);  
    $stmt->bindParam(':password', $password);  
    $stmt->bindParam(':admin', $admin);  
		$stmt->execute();
	}