PHP validate if age input into registration form is over 18 [duplicate]

I’m trying to check if the birthday the user inputs in my html form is over 18 years.

function validate_form() {
    $errors = array(); 
    $insert = array();

    # stores date of birth.
    $insert['bday'] = array_key_exists('bday', $_REQUEST) ? $_REQUEST['bday'] : null;

    # this is line 66
    if  (date('Y') - $insert['bday'] <= 18)  {
        $errors[] = "<p> You need to be 18 years or older to create an account.</p>n";
    }

If for example the user chooses 2003 and below, the form submits successfully. if the user selects 2005 and above the error message displays as it should. However if i select 09/01/2004 (which was more than 18 years ago now) i still get the error message.

I also get this warning when the error page displays:

Warning: A non-numeric value encountered in C:xampphtdocsfilnamefilenamepagename.php on line 66