Lo hice así:
y funciona de 10, gracias!
Código (php) [Seleccionar]
<?php
if(isset($_POST['select_day_birth']))
{
?>
<script type="text/javascript">
$(document).ready(function()
{
var select_day = document.getElementById('select_day_birth');
var op_day = select_day.getElementsByTagName('option');
op_day[<?php echo($_POST['select_day_birth']); ?>].selected = true;
});
</script>
<?php
}
?>
y funciona de 10, gracias!