<form action="photo.php?data=true" id="formPhoto" method="GET">
Показать фотографии за дату
<input class="datepicker-here" name="data" required=""></input>
<button type="submit" class="btn photorbut" >Показать</button>
</form>
required=""
- некорректный синтаксис<!-- HTML: --> <input class="datepicker-here" name="data" required></input>
<!-- XHTML: --> <input class="datepicker-here" name="data" required="required"></input>