This question has been answered by whitebyte. See the first response.
<?php
$arr = $modx->sanitize($_POST);
array_pop($arr);
echo "<pre>";
print_r($arr);
echo "</pre>";
$newarray = array();
foreach ($arr as $k => $v) {
$newarray[] = "<FL val='$k'>$v</FL>";
}
$data = implode(',', $newarray);
var_dump($newarray);