Ztracená hodnota POST v mysql_real_escape_string

<?php
if (isset($_POST['submit'])) {
    print_r($_POST); // Array ( [name] => aaa [mail] => aaa@bbb.cc )
    echo '<br>';
    print_r($_POST['name']); // aaa
    echo '<br>';
    $a=("    UPDATE users
            SET name = '" . mysql_real_escape_string($_POST['name']) . "',
                mail = '" . mysql_real_escape_string($_POST['mail']) . "'");

    echo $a; // UPDATE users SET name = '', mail = '' // PROČ JSOU HODNOTY PRÁZDNÉ?

    $a=("    UPDATE users
            SET name = '" . $_POST['name'] . "',
                mail = '" . $_POST['mail'] . "'");

    echo $a; // UPDATE users SET name = 'aaa', mail = 'aaa@bbb.cc'
}
exit;