try {
  $sql = <<<'SQL'
SELECT
    T.MON$TRANSACTION_ID AS TRANSACTION_ID,
    T.MON$TIMESTAMP AS START_DATE,
    TRIM(CASE
      WHEN T.MON$ISOLATION_MODE = 0 THEN 'CONSISTENCY'
      WHEN T.MON$ISOLATION_MODE = 1 THEN 'CONCURRENCY'
      WHEN T.MON$ISOLATION_MODE = 2 THEN 'READ COMMITTED RECORD VERSION'
      WHEN T.MON$ISOLATION_MODE = 3 THEN 'READ COMMITTED NO RECORD VERSION'
      WHEN T.MON$ISOLATION_MODE = 4 THEN 'READ COMMITTED READ CONSISTENCY'
    END) AS ISOLATION_MODE,
    TRIM(CASE
      WHEN T.MON$LOCK_TIMEOUT = -1 THEN 'Infinite wait'
      WHEN T.MON$LOCK_TIMEOUT = 0 THEN 'No wait'
      WHEN T.MON$LOCK_TIMEOUT > 0 THEN 'Timeout ' || MON$LOCK_TIMEOUT
    END) AS LOCK_TIMEOUT,
    TRIM(CASE
      WHEN T.MON$READ_ONLY = 1 THEN 'READ'
      ELSE 'READ WRITE'
    END) AS READ_ONLY,
    (T.MON$AUTO_COMMIT = 1) AS AUTO_COMMIT,
    (T.MON$AUTO_UNDO = 1) AS AUTO_UNDO
FROM MON$TRANSACTIONS T  
WHERE T.MON$TRANSACTION_ID = CURRENT_TRANSACTION
SQL;  
	
  $db = new PDO($dbname, $dbuser, $dbpass,  [PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION]);
  $db->setAttribute(PDO::ATTR_AUTOCOMMIT, false);
  $db->exec("SET TRANSACTION READ ONLY ISOLATION LEVEL READ COMMITTED RECORD_VERSION NO WAIT");
  $query = $db->prepare($sql);
  $query->execute();
  $row = $query->fetchObject();
  $query->closeCursor();
  $db->exec("COMMIT");
  echo '<pre>';
  var_dump($row);
  echo '</pre>';
}
catch(Throwable $e) {
	echo $e->getMessage();
}

object(stdClass)#3 (7) {
  ["TRANSACTION_ID"]=>
  int(34418)
  ["START_DATE"]=>
  string(19) "2021-05-25 23:06:51"
  ["ISOLATION_MODE"]=>
  string(29) "READ COMMITTED RECORD VERSION"
  ["LOCK_TIMEOUT"]=>
  string(7) "No wait"
  ["READ_ONLY"]=>
  string(4) "READ"
  ["AUTO_COMMIT"]=>
  bool(false)
  ["AUTO_UNDO"]=>
  bool(true)
}