計算與生日相差天數

$customer_dob = '1975-09-21';
$temp_dob = explode("-",$customer_dob);
$temp_dob = mktime(0,0,0,$temp_dob[1],$temp_dob[2],date('Y', time()));
$temp_dob = round(($temp_dob - time())/3600/24); //與生日相差天數