<p><input type="checkbox" name="list[]" value="кто-нибудь"> кто-нибудь</p>
<p><input type="checkbox" name="list[]" value="еще кто-нибудь"> еще кто-нибудь</p>
$name = $_POST['name'];
$list = implode(",", $_POST['list']);
$query = "INSERT INTO `conversatons` VALUES ('$name', '$list'); ";
$day1 = mktime(0, 0, 0, date("m"), date("d")-7, date("Y"));
$day2 = mktime(0, 0, 0, date("m"), date("d")-6, date("Y"));
$sql = "SELECT * FROM table WHERE UNIX_TIMESTAMP(createdon) > $day1 AND UNIX_TIMESTAMP(createdon) < $day2";
$result = mysql_query($sql);
$data = array();
while($row = mysql_fetch_assoc($result))
{
if(!isset($data[$row['email']])){
$data[$row['email']] = array();
}
$data[$row['email']][] = $row;
}
var_dump($data);