create table table_name(id int primary key,register_time int(10));

/* 查询2015-12-01 至 2015-12-14 */
// 开始的时间戳
$startUnix = 1448899200; // 2015-12-01 00:00:00
// 结束的时间戳
$endUnix = 1450108800; // 2015-12-15 00:00:00
for($i = $startUnix; $i < $endUnix; $i += 86400){ // 86400为1天的秒数
 // 查询
 $sql = 'select count(*) from table_name where register_time>= '.$i.' and register_time < '.$i + 86400;
 // 执行查询
}

/* 查询2015-12-01 至 2015-12-14 */
// 开始的时间戳
$startUnix = 1448899200; // 2015-12-01 00:00:00
// 结束的时间戳
$endUnix = 1450108800; // 2015-12-15 00:00:00
$sql = 'select count(id) as register_count, FROM_UNIXTIME(register_time, '%Y-%m-%d') as datetime from table_name where register_time>= '.$startUnix.' and register_time < '.$endUnix group by datetime;
// 执行查询
...