将日期数组插入MySQL

I am coding a kind of simple registration form for a company for employees to select the dates the will be available for work. I am having trouble when a user selects multiple dates, it writes 0000-00-00 in the database. I would like it to make two records of both dates.

This is my basic form code:

<form method="post" action="action.php">
<input name="UserID" type="hidden" value="3">
<input type="checkbox" name="Datum[]" value="2014-01-04">4 january
<input type="checkbox" name="Datum[]" value="2014-01-06">6 january
<input name="submit" type="submit" />
</form>

My action.php looks like this:

<?php

include("config.php");

$UserID=$_POST['UserID'];
$Datum=$_POST['Datum'];


?>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Test</title>
</head>

<body>

 <?php 

       $res = mysql_query("insert into workingdays (UserID, Datum) 
                 values ('$UserID', '" . implode(',', $_POST['Datum']) ."')");

                if(!$res){
                echo "Error! <a href='/mysql'>Go back.</a>";
                }
                else {
                echo "Succesful. <a href='/mysql'>Go back.</a>";
                }

         ?>


</body>
</html>

I use two tables.

wp_users (the actual wordpress table):

ID | display_name | Email

workingdays:

ID | UserID | Date

The UserID of 'workingdays' and the ID of wp_users are joined. I would like to see every option of the checkbox get its own row.

How can I achieve this?

First foreach the $_POST['Datum'] then mysql insert.

foreach ($_POST['Datum'] as $date){
   $res = mysql_query("insert into workingdays (UserID, Date) 
            values ('$UserID', '$date')");

            if(!$res){
            echo "Error! <a href='/mysql'>Go back.</a>";
            }
            else {
            echo "Succesful. <a href='/mysql'>Go back.</a>";
            }
}

I hope this is what you meant..

If its only a couple fields per user, use http://codex.wordpress.org/Function_Reference/update_user_meta and http://codex.wordpress.org/Function_Reference/get_user_meta

which uses user_meta table based on user ID