Неверно записывает с формы в MySQL

    <div class="new_contract" align="center" >
    <div id="inputsys" align="center">Добавление нового контракта</div>
    <div class="new_contract_form">
    <form id="add_new_contract_form" action="add_new_contract_from.php" method = "POST" >
    <label for = "reg_contract">Номер контракта</label> <br />
    <input type = "text" maxlength ="10" required name ="reg_contract" class="input-field" /> <br />
    </div>
    <input type = "submit" id = "submit" name = "submit" class="supper-button" value="Отправить" />
  <?php
     include 'config.php';
    $id = $_GET[id];
    $reg_contract = $_POST['reg_contract'];
    $query = "INSERT INTO `BankClient`.`Contract` (`user_ID`, `Contract_number`) VALUES ('".$id."','".$reg_contract."')";
    $result = mysql_query ($query);
?>

<a href="add_new_contract_from.php?id='.$data['id'].'

$data['user_id'] = "'".$id = $_GET['id']."'";
$data['contract_number'] = "'".$_POST['reg_contract']."'";
$data = implode(',',$data);
 
$query = "INSERT INTO `BankClient`.`Contract` (`user_ID`, `Contract_number`) VALUES($data)";

<div class="new_contract" align="center" >
    <div id="inputsys" align="center">Добавление нового контракта</div>
    <form id="add_new_contract_form" action="" method = "POST" >
        <div class="new_contract_form">
            <label for = "reg_contract">Номер контракта</label> <br />
            <input type = "text" maxlength ="10" required name ="reg_contract" class="input-field" /> <br />
        </div>
        <input type="submit" id="submit" name="submit" class="supper-button" value="Отправить" />
    </form>
<?php
    include 'config.php';
    
    # Если нажата кнопка "Отправить"
    if(isset($_POST["submit"]))
    {
        $id = (int) $_GET['id'];
        $reg_contract = mysql_real_escape_string( $_POST['reg_contract'] );
        $query = "INSERT INTO `BankClient`.`Contract` (`user_ID`, `Contract_number`) VALUES ('".$id."','".$reg_contract."')";
        $result = mysql_query ($query);
    }
?>

if(isset($_POST["submit"]))