- Edited
If is correct this validation
$age = $_POST["age"];
$onlynumbersage = preg_replace("/(19[0-9][0-9]|20[0-9][0-9]){4}-[01-12]{2}-[01-31]{2}/","", $age);
if(date_parse_from_format("Y-n-j",$onlynumbersage )){
mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$link = mysqli_connect("localhost", "root", "P-11fl32fg14", "check-in");
$query = "INSERT INTO check-in (age) VALUES ('$onlynumbersage ')";
echo $query;
mysqli_query($link, $query);
mysqli_close($link);
}
elseif(!empty($onlynumbersage )){
echo "you should enter one age 09-04-1984.";
}