PHP - Final input-validate - php

Question

i am validating three types of input(string,email,url):
string-validating:
if ($_POST['string'] != "") {
$string = filter_var($_POST['string'], FILTER_SANITIZE_STRING);
if ($string != "") {
// valid
} else {
// not valid
}
} else {
// empty
}
email-validating:
if ($_POST['email'] != "") {
$email = filter_var($_POST['email'], FILTER_SANITIZE_EMAIL);
if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
// valid
} else {
// not valid
} else {
// empty
}
url-validating:
if ($_POST['url'] != "") {
$url = filter_var($_POST['url'], FILTER_SANITIZE_URL);
if (filter_var($url, FILTER_VALIDATE_URL)) {
// valid
} else {
// not valid
}
} else {
// empty
}
After doing this checks i use PDO - Prepared statements to inserate in database.
You think this is secure enough or did i missed some points?
Hope for your answers, thanks and greetings!