$sql = "SELECT id, firstName, middleName, lastName FROM users_info";
$result = $pdo->query($sql);

$rows = array();

while($row = $result->fetch()){
     $time = $pdo->query(
                "SELECT CAST(SUM(timediff(dateTimeOut, dateTimeIn))
                 as time) as totalHoursWorked
                 FROM users_time WHERE user_id = {$row['id']}"
            )->fetch();

     $row['totalHoursWorked'] = $time['totalHoursWorked'];
     $rows[] = $row;

}

echo json_encode($rows);

"SELECT CAST(SUM(timediff(dateTimeOut, dateTimeIn))
                 as time) as totalHoursWorked
                 FROM users_time WHERE user_id = {$row['id']}"

SELECT
    id
  , firstName
  , middleName
  , lastName
  , CAST(SUM(timediff(dateTimeOut, dateTimeIn))
                     as time) as totalHoursWorked
FROM users_info
INNER JOIN users_time ON user_id = id;