Having trouble with a foreach   mysql INSERT
<script>
var count = 0;
$(function(){
$('p#add_field').click(function(){
    count += 1;
    $('#container').append(
            '<strong>Recipient #' + count + '</strong><br />'
            + '<tr><td><label>Client First Name:</label><input id="f_name_' + count + '" name="f_name[]' + '" type="text" /></td><td><label>Client Last Name:</label><input id="l_name_' + count + '" name="l_name[]' + '" type="text" /></td></tr>');

});
});
</script>

//Loop through added fields
foreach ( $_POST['f_name'] as $key=>$f_name) {
  foreach ( $_POST['l_name'] as $key=>$l_name) {

    //Insert static values into table
    $sql_user = sprintf("INSERT INTO mytable (f_name, l_name) VALUES ('%s','%s')",
    mysql_real_escape_string($f_name),
    mysql_real_escape_string($l_name));
    $result_user = $db->query($sql_user);
    }
    }


<?php if (!isset($_POST['btnSubmit'])) { ?>
<h1>My Form</h1>
    <form name="userdata" method="post" action="">

    <h3>Please list the names:</h3>
<table>
    <div id="container">
        <p id="add_field"><a href="#"><span>+ Add Names</span></a></p>
    </div>
</table>

    <input type="submit" name="btnSubmit" id="go" value="Submit Your Names" class="btn" />


    </form>
<?php } ?>

foreach ( $_POST['f_name'] as $key => $f_name ) {
    $l_name = $_POST['l_name'][$key];
    // do something with $f_name and $l_name
}