Validating Date of Birth format from a form field value
Would it be worth doing some client-side validation first using javascript to check the date of birth field is entered correctly? You can use an onChange event on your field and then split the string on "/" and check each part is within a valid range 0-31, 1-12, 1900-2016.

For serverside checking, you can use strtotime() and check that the return isn't false.