如何判斷字串的年月日？（問題更新）

Question

$rangeY = range(1950, 2015); $rangeM = range(1, 12); $rangeD = range(1, 31); <select class="profile_birthday_select" name="yy" id="yy"> <?foreach ($rangeY as $key) {?> <option value="male"><?=$key;?></option> <?}?> </select> <select class="profile_birthday_select" name="mm" id="mm"> <?foreach ($rangeM as $key) {?> <option value="male"><?=$key;?></option> <?}?> </select> <select class="profile_birthday_select" name="dd" id="dd"> <?foreach ($rangeD as $key) {?> <option value="male"><?=$key;?></option> <?}?> </select> 我想判斷該用戶的個別年月日但是過來的數據會是這樣的格式：String且是 YYYY-MM-DD <?=$data->{'birthday'};?> 這樣我要怎麼個別取出YYYY、MM、ＤＤ？