Actually all MySQL database operations are made twice.
It doesn't matter (except for performance and load)
for select queries, but it has much todo with session data.

First time my function writes the right session data
and second time some of the session data gets lost (not always, but randomly).

I tried to output written data to the screen in _write
function and it differs sometimes from the data being read
in _read function. It was very confusing, because the
second thread does not output anything to the screen.
So I started MySQL with --log parameter to see, what 
commands are sent to the database and saw, that
2 threads are reading and writing to the database. 
(thread 12 writes correct data and thread 11 wrong data,
Although I am the only user in the system)

Maybe it has something todo with 2 processor system?

My theory is also backed up by GET_LOCK function.
When I acquire lock in _write function, the second thread
probably fails to write to the database and all session data gets written right.


I use session class:

<?php
class Session {
  
  var $name;
  var $id;
  var $split_len;
  var $table_name;
  var $db_name;
  var $ttl;
  var $tmp_read;

  function Session($_session_name = 'SESSID', $_ttl = 900, $_use_cookies = 0, $_gc_percent = 1){
    $this->name = $_session_name;
    $this->split_len = 256;
    $this->ttl = $_ttl; //default session time to live is 15 min -> 900 seconds.
    $this->table_name = 'session';
    $this->db_name = 'user';
    session_name($this->name);
    ini_set('session.use_cookies', $_use_cookies);
    ini_set('session.gc_probability', $_gc_percent);
    ini_set('session.use_trans_sid',0);
    @mysql_select_db($this->db_name);
    session_set_save_handler(
      array(&$this, "_sess_open"),
      array(&$this, "_sess_close"),
      array(&$this, "_sess_read"),
      array(&$this, "_sess_write"),
      array(&$this, "_sess_destroy"),
      array(&$this, "_sess_gc")
      );
    session_start();
    $this->id = session_id();
  }

  function set_db($_db, $_table){
    $this->db_name = $_db;
    $this->table_name = $_table;
  }

  function _sess_open($_spath, $_sname){
    @mysql_select_db($this->db_name);
    return true;  
  }

  function _sess_read($_id){
    @mysql_select_db($this->db_name);
    $res = @mysql_query('SELECT data, UNIX_TIMESTAMP(changed) AS changed FROM '.$this->table_name.' WHERE name="'.$this->name.'" AND id="'.$_id.'" AND (UNIX_TIMESTAMP(changed)+'.$this->ttl.') >= '.time().' ORDER BY pos');
    $ret_str = '';
    while ($row = @mysql_fetch_object($res)) {
      $ret_str .= $row->data; 
      }

    $ret_str = (string)base64_decode($ret_str);
    
    return (string)$ret_str;
  }

  function _sess_write($_id, $_data){
    $_data = base64_encode($_data);
    @mysql_select_db($this->db_name);
//    mysql_query("SELECT GET_LOCK('".$_id."',120)");
    @mysql_query('DELETE FROM '.$this->table_name.' WHERE name="'.$this->name.'" AND id="'.$_id.'"');
    $pos = 0;
    
    while ($part = substr($_data, ($pos * $this->split_len), $this->split_len)) {
      mysql_query('INSERT INTO '.$this->table_name.'(id, name, pos, data) VALUES("'.$_id.'","'.$this->name.'",'.($pos++).',"'.$part.'")');
      } //while

    return true;
  }

  function _sess_close(){
    return true;
  }

  function _sess_destroy($_id){
    @mysql_query('DELETE FROM '.$this->table_name.' WHERE name="'.$this->name.'" AND id="'.$_id.'"');
    return true;
  }

  function _sess_gc($_max_lifetime){
    @mysql_query('DELETE FROM '.$this->table_name.' WHERE name="'.$this->name.'" AND (UNIX_TIMESTAMP(changed)+'.$this->ttl.') < '.time());
    return true;
  }

} //class Session

$sess = new Session('MUMM', 900, 0, 1);
?>

MySQL log:

020907 22:04:02	     11 Statistics 
		     12 Connect     suser@localhost on 
		     11 Init DB     user
		     11 Init DB     user
		     11 Init DB     user
		     11 Query       SELECT data, UNIX_TIMESTAMP(changed) AS changed FROM session WHERE name="MUMM" AND id="ecb7cbefe334e74f55de53d5875136ef" AND (UNIX_TIMESTAMP(changed)+900) >= 1031429042 ORDER BY pos
		     11 Init DB     user
		     11 Query       SELECT md5 as md, txt_ee AS txt FROM i18n
		     12 Init DB     user
		     12 Init DB     user
		     12 Init DB     user
		     12 Query       SELECT data, UNIX_TIMESTAMP(changed) AS changed FROM session WHERE name="MUMM" AND id="ecb7cbefe334e74f55de53d5875136ef" AND (UNIX_TIMESTAMP(changed)+900) >= 1031429042 ORDER BY pos
		     12 Init DB     user
		     12 Query       SELECT md5 as md, txt_ee AS txt FROM i18n
		     12 Init DB     user
		     12 Query       DELETE FROM session WHERE name="MUMM" AND id="ecb7cbefe334e74f55de53d5875136ef"
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",0,"c2J5fHM6MjoiMTYiO3N3fHM6MDoiIjtldnxhOjE5OntzOjM6ImtleSI7czoyOiI3NiI7czo0OiJuaW1pIjtzOjk6IkVTVElLTyBBUyI7czo2OiJlcmVnbnIiO3M6ODoiMTAwMDAzNTYiO3M6MToidCI7czoxOiJKIjtzOjQ6ImFkZHIiO3M6MjA6IlNvb2xhIDggNTAxMDcgIFRhcnR1IjtzOjM6InRlbCI7czo5OiIwNy8zNzEwMDAiO3M6Mzoi")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",1,"ZmF4IjtzOjk6IjA3LzM3MTAxMCI7czo3OiJrYXBpdGFsIjtzOjE0OiI1OSA4NTAgNjYwIEVFSyI7czo0OiJtYWlsIjtzOjE2OiJlc3Rpa29AZXN0aWtvLmVlIjtzOjM6Ind3dyI7czoxMzoid3d3LmVzdGlrby5lZSI7czo2OiJhcmlyZWciO3M6Mjg6IjA5LjA0LjE5OTYgVGFydHUgxHJpcmVnaXN0ZXIiO3M6NDoidm9ybSI7czoxMToiQWt0")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",2,"c2lhc2VsdHMiO3M6MzoidGVnIjthOjI6e2k6MDtzOjQ3OiJJc2lrdWxlIGt1dWx1dmF0ZSBtaXR0ZWVsdXJ1dW1pZGUgduRsamH8/HJpbWluZSI7aToxO3M6NDY6Iktpbm5pc3ZhcmEgaGFsZHVzIHRhc3UgZWVzdCB29WkgbGVwaW5ndSBhbHVzZWwiO31zOjM6InJhcCI7YToxOntzOjE6IkUiO2E6MTp7aTowO2E6Mjp7aTowO3M6MTU6IjEw")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",3,"MDAwMzU2X0UuaHRtbCI7aToxO3M6MTA6IjI4LjA5LjIwMDAiO319fXM6MjoiYmsiO3M6MTA6IjI2LjA4LjIwMDIiO3M6MzoiYmlsIjthOjQ6e2k6MDtzOjM6IlMwMSI7aToxO3M6MzoiUzAwIjtpOjI7czozOiJTOTkiO2k6MztzOjM6IlM5OCI7fXM6NDoidmFsdiI7czowOiIiO3M6MjoibWgiO3M6MDoiIjtzOjM6Im1oYyI7aTowO31rbHxO")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",4,"O3J0aW1lfE47cGdnfGk6MDtjdGltZXxpOjEwMzE0Mjg2MjY7bWFzc3xhOjIyOntpOjE0O2E6Mjp7czoxOiJlIjtzOjg6IjEwMDc0NTI1IjtzOjE6Im4iO3M6MjU6IkEgJiBMIE1BTkFHRU1FTlQgRUVTVEkgQVMiO31pOjIxO2E6Mjp7czoxOiJlIjtzOjg6IjEwNTU5NTMwIjtzOjE6Im4iO3M6OToiQUFCRUxJIE/cIjt9aToyNjthOjI6e3M6")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",5,"MToiZSI7czo4OiIxMDA0NDAzMyI7czoxOiJuIjtzOjg6IkFMREVSIE/cIjt9aTozNzthOjI6e3M6MToiZSI7czo4OiIxMDQ1Nzg0MSI7czoxOiJuIjtzOjE1OiJBTUVSIFRPQkFDQ08gQVMiO31pOjQ4O2E6Mjp7czoxOiJlIjtzOjg6IjEwNzAxODczIjtzOjE6Im4iO3M6MjI6IkFVU1RSSUEgVEFCQUsgRUVTVEkgT9wiO31pOjYzO2E6Mjp7")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",6,"czoxOiJlIjtzOjg6IjEwMzEyMDgzIjtzOjE6Im4iO3M6MTg6IkNPRkZJSyAmIFBVRkZFVCBP3CI7fWk6NzY7YToyOntzOjE6ImUiO3M6ODoiMTAwMDAzNTYiO3M6MToibiI7czo5OiJFU1RJS08gQVMiO31pOjg2O2E6Mjp7czoxOiJlIjtzOjg6IjEwMzc2OTMwIjtzOjE6Im4iO3M6MjQ6IkhPVVNFIE9GIFBSSU5DRSBFRVNUSSBBUyI7fWk6")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",7,"OTQ7YToyOntzOjE6ImUiO3M6ODoiMTA4MTM5MDAiO3M6MToibiI7czoxMToiS0FSSVBMT04gT9wiO31pOjEwMTthOjI6e3M6MToiZSI7czo4OiIxMDI1NjEzNyI7czoxOiJuIjtzOjE1OiJLUkVESUlESUlORk8gQVMiO31pOjExMjthOjI6e3M6MToiZSI7czo4OiIxMDE5MzEwNiI7czoxOiJuIjtzOjE5OiJMT1NTSSBLSU5OSVNWQVJBIE/c")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",8,"Ijt9aToxMjc7YToyOntzOjE6ImUiO3M6ODoiMTA2NzUyNTkiO3M6MToibiI7czoxOToiTUVSSURJQU5VUyBERUxUQSBP3CI7fWk6MTMyO2E6Mjp7czoxOiJlIjtzOjg6IjEwNDUzNjA3IjtzOjE6Im4iO3M6MTQ6Ik1VTFRJTElOR1VBIEFTIjt9aToxNDU7YToyOntzOjE6ImUiO3M6ODoiMTAzOTcxMjIiO3M6MToibiI7czo2OiJPUk0gT9wi")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",9,"O31pOjE2MDthOjI6e3M6MToiZSI7czo4OiIxMDA3ODY4NyI7czoxOiJuIjtzOjg6IlJFTUVUIEFTIjt9aToxNzU7YToyOntzOjE6ImUiO3M6ODoiMTA1MjY3NzIiO3M6MToibiI7czoyNToiUkVWSVZBTCBTVk9SSVMgRVNUT05JQSBP3CI7fWk6MTg3O2E6Mjp7czoxOiJlIjtzOjg6IjEwNTg2ODczIjtzOjE6Im4iO3M6MTM6IlJFVklWQUxM")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",10,"LVYgT9wiO31pOjIwMDthOjI6e3M6MToiZSI7czo4OiIxMDA2Njg0NSI7czoxOiJuIjtzOjExOiJTRVJFTkRBTCBBUyI7fWk6MjEyO2E6Mjp7czoxOiJlIjtzOjg6IjEwODcyNDk3IjtzOjE6Im4iO3M6MTM6IlNLVlMgR1JVUFAgT9wiO31pOjIyMzthOjI6e3M6MToiZSI7czo4OiIxMDEwMDI0MCI7czoxOiJuIjtzOjEyOiJTWUxWRVNURVIg")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",11,"QVMiO31pOjIyOTthOjI6e3M6MToiZSI7czo4OiIxMDM4NzI3NiI7czoxOiJuIjtzOjE3OiJUUkFOU0ZFUiBHUlVQUCBP3CI7fWk6MjQyO2E6Mjp7czoxOiJlIjtzOjg6IjEwNzM1NDI5IjtzOjE6Im4iO3M6MTI6IlfVUk8gVkFSQSBP3CI7fX1hdXRofE86NDoiYXV0aCI6OTp7czo2OiJrbGllbnQiO2E6Nzp7czo0OiJuaW1pIjtzOjE0OiJW")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",12,"bGFkaW1pciBS/G50/CI7czoxMToia2xpZW5kaWtvb2QiO3M6MToiMSI7czoxMDoib25saW5la29vZCI7czo0OiIxODM0IjtzOjU6ImVtYWlsIjtzOjIxOiJ2bGFkc0BrcmVkaWlkaWluZm8uZWUiO3M6NjoidHVubnVzIjtzOjE6IksiO3M6NzoiYnJvd3NlciI7czo2OiJpZS5naWYiO3M6NjoiZXJlZ25yIjtzOjg6IjEwMjU2MTM3Ijt9czo3")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",13,"OiJrZXJlZ25yIjtOO3M6NToibG9naW4iO047czo3OiJpc19hdXRoIjtiOjE7czo0OiJfdHRsIjtpOjgwMDtzOjU6Il90aW1lIjtpOjEwMzE0MjkwNDI7czo4OiJpbmZvX3R4dCI7czowOiIiO3M6ODoibmV4dF9ybmQiO047czo4OiJsb2dpbl9pZCI7Tjt9aGlzdHxhOjU6e2k6MDthOjI6e3M6MToiZSI7czo4OiIxMDA3NDUyNSI7czoxOiJu")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",14,"IjtzOjI1OiJBICYgTCBNQU5BR0VNRU5UIEVFU1RJIEFTIjt9aToxO2E6Mjp7czoxOiJlIjtzOjg6IjEwMDAwMzU2IjtzOjE6Im4iO3M6OToiRVNUSUtPIEFTIjt9aToyO2E6Mjp7czoxOiJlIjtzOjg6IjEwNDU3ODQxIjtzOjE6Im4iO3M6MTU6IkFNRVIgVE9CQUNDTyBBUyI7fWk6MzthOjI6e3M6MToiZSI7czo4OiIxMDMxMjA4MyI7czox")
		     12 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",15,"OiJuIjtzOjE4OiJDT0ZGSUsgJiBQVUZGRVQgT9wiO31pOjQ7YToyOntzOjE6ImUiO3M6ODoiMTA4MTM5MDAiO3M6MToibiI7czoxMToiS0FSSVBMT04gT9wiO319aXZ8YTowOnt9")
		     12 Query       ROLLBACK
		     11 Init DB     user
		     11 Query       DELETE FROM session WHERE name="MUMM" AND id="ecb7cbefe334e74f55de53d5875136ef"
		     11 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",0,"c2J5fGk6NTtzd3xzOjA6IiI7ZXZ8YToxOTp7czozOiJrZXkiO3M6MjoiNzYiO3M6NDoibmltaSI7czo5OiJFU1RJS08gQVMiO3M6NjoiZXJlZ25yIjtzOjg6IjEwMDAwMzU2IjtzOjE6InQiO3M6MToiSiI7czo0OiJhZGRyIjtzOjIwOiJTb29sYSA4IDUwMTA3ICBUYXJ0dSI7czozOiJ0ZWwiO3M6OToiMDcvMzcxMDAwIjtzOjM6ImZheCI7")
		     11 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",1,"czo5OiIwNy8zNzEwMTAiO3M6Nzoia2FwaXRhbCI7czoxNDoiNTkgODUwIDY2MCBFRUsiO3M6NDoibWFpbCI7czoxNjoiZXN0aWtvQGVzdGlrby5lZSI7czozOiJ3d3ciO3M6MTM6Ind3dy5lc3Rpa28uZWUiO3M6NjoiYXJpcmVnIjtzOjI4OiIwOS4wNC4xOTk2IFRhcnR1IMRyaXJlZ2lzdGVyIjtzOjQ6InZvcm0iO3M6MTE6IkFrdHNpYXNl")
		     11 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",2,"bHRzIjtzOjM6InRlZyI7YToyOntpOjA7czo0NzoiSXNpa3VsZSBrdXVsdXZhdGUgbWl0dGVlbHVydXVtaWRlIHbkbGph/PxyaW1pbmUiO2k6MTtzOjQ2OiJLaW5uaXN2YXJhIGhhbGR1cyB0YXN1IGVlc3QgdvVpIGxlcGluZ3UgYWx1c2VsIjt9czozOiJyYXAiO2E6MTp7czoxOiJFIjthOjE6e2k6MDthOjI6e2k6MDtzOjE1OiIxMDAwMDM1")
		     11 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",3,"Nl9FLmh0bWwiO2k6MTtzOjEwOiIyOC4wOS4yMDAwIjt9fX1zOjI6ImJrIjtzOjEwOiIyNi4wOC4yMDAyIjtzOjM6ImJpbCI7YTo0OntpOjA7czozOiJTMDEiO2k6MTtzOjM6IlMwMCI7aToyO3M6MzoiUzk5IjtpOjM7czozOiJTOTgiO31zOjQ6InZhbHYiO3M6MDoiIjtzOjI6Im1oIjtzOjA6IiI7czozOiJtaGMiO2k6MDt9a2x8TjtydGlt")
		     11 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",4,"ZXxOO3BnZ3xpOjA7Y3RpbWV8aToxMDMxNDI4NjI2O21hc3N8YTowOnt9YXV0aHxPOjQ6ImF1dGgiOjk6e3M6Njoia2xpZW50IjthOjc6e3M6NDoibmltaSI7czoxNDoiVmxhZGltaXIgUvxudPwiO3M6MTE6ImtsaWVuZGlrb29kIjtzOjE6IjEiO3M6MTA6Im9ubGluZWtvb2QiO3M6NDoiMTgzNCI7czo1OiJlbWFpbCI7czoyMToidmxhZHNA")
		     11 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",5,"a3JlZGlpZGlpbmZvLmVlIjtzOjY6InR1bm51cyI7czoxOiJLIjtzOjc6ImJyb3dzZXIiO3M6NjoiaWUuZ2lmIjtzOjY6ImVyZWduciI7czo4OiIxMDI1NjEzNyI7fXM6Nzoia2VyZWduciI7TjtzOjU6ImxvZ2luIjtOO3M6NzoiaXNfYXV0aCI7YjoxO3M6NDoiX3R0bCI7aTo4MDA7czo1OiJfdGltZSI7aToxMDMxNDI5MDQyO3M6ODoiaW5m")
		     11 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",6,"b190eHQiO3M6MDoiIjtzOjg6Im5leHRfcm5kIjtOO3M6ODoibG9naW5faWQiO047fWhpc3R8YTo1OntpOjA7YToyOntzOjE6ImUiO3M6ODoiMTAwNzQ1MjUiO3M6MToibiI7czoyNToiQSAmIEwgTUFOQUdFTUVOVCBFRVNUSSBBUyI7fWk6MTthOjI6e3M6MToiZSI7czo4OiIxMDAwMDM1NiI7czoxOiJuIjtzOjk6IkVTVElLTyBBUyI7fWk6")
		     11 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",7,"MjthOjI6e3M6MToiZSI7czo4OiIxMDQ1Nzg0MSI7czoxOiJuIjtzOjE1OiJBTUVSIFRPQkFDQ08gQVMiO31pOjM7YToyOntzOjE6ImUiO3M6ODoiMTAzMTIwODMiO3M6MToibiI7czoxODoiQ09GRklLICYgUFVGRkVUIE/cIjt9aTo0O2E6Mjp7czoxOiJlIjtzOjg6IjEwODEzOTAwIjtzOjE6Im4iO3M6MTE6IktBUklQTE9OIE/cIjt9fWl2")
		     11 Query       INSERT INTO session(id, name, pos, data) VALUES("ecb7cbefe334e74f55de53d5875136ef","MUMM",8,"fGE6MDp7fQ==")
		     11 Query       ROLLBACK
		     11 Quit