у тебя правильно организована транзакция, одна на все инсерты
foreach($Data as $items)
{
$query = "INSERT INTO employee (ssn,name,phone) values ($items['ssn'],$items['name'],$items['phone'])";
};
mysql_connect(....) ;
mysql_select_db("mytable");
$Data = file_get_contents("data.json");
$object = json_decode($Data);
/* Начало транзакции */
$mysqli->begin_transaction();
try {
foreach($Data as $items)
{
$query = "INSERT INTO employee (ssn,name,phone) values ($items['ssn'],$items['name'],$items['phone'])";
};
$mysqli->commit();
} catch (mysqli_sql_exception $exception) {
$mysqli->rollback();
throw $exception;
}