Why don't you just do this:
$hour = date('H');
$minute = (date('i')>30)?'30':'00';
echo "$hour:$minute";
DEMO
UPDATE
I just realized that you asked all time to be rounded to :30
:
$hour = date('H');
$minute = '30'; //always 30
echo "$hour:$minute";
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…