Can I do a warning validate from Datepicker and alert swal like this?

I do that and it error that is_string is not defined.

I'm new in php btw

The comment is a think that i tried to make in different way sorry abort that.

form

<div class="form-group">
<label>Birthdate</label>
    <div class="input-group date">
    <span class="input-group-addon"><i class="fa fa-calendar"></i></span><input type="text" class="form-control datepicker_class" id="birth_date" name="birth_date" onchange="validateAge">
    </div>
</div>

function

function validateAge(){
var birthday = new $('#birth_date').val();
// let birthday = $('#birth_date').val();

// check = explode('/',birthday);
birthday = explode("/", birthday);
// alert(check);

if (is_string(birthday)) {
    birthday = strtotime(birthday);
}

// check
// 31536000 is the number of seconds in a 365 days year. strtotime for 1 years : 31556926 
if(time() - birthday < 18 * 31556926)  {
    swal({
            title: "warning",
            text: "your age is lower than 18",
            type: "warning"
        });
}

return true;
}