try changing this code to:
if(isset($_POST['DOB'])) {
$_POST['DOB'] = date("Y-m-d", strtotime(str_replace("/","-",$_POST['DOB'] )));
}
to:
if(isset($_POST['DOB']) && $_POST['DOB'] != "") {
$_POST['DOB'] = date("Y-m-d", strtotime(str_replace("/","-",$_POST['DOB'] )));
}