<?php
function inverteData($data, $separar = '-', $juntar = '-'){
        return implode($juntar, array_reverse(explode($separar, $data)));
}

$data = $_POST['data']; //14/02/2011
$hora = $_POST['hora']; //12:00;
$novaData = inverteData($data, '/', '-');
$novaHora = $hora.':00'; //Acrescenta os segundos
$dataHora = "$novaData $novaHora"; //saída: 2011-02-14 12:00:00