OK, realised that of course the $SQL -> error is set (when it is set) right after multi_query() is called. So that should be :

if ($SQL -> multi_query(...))
{
   do { ... } while (...);
}
else
  echo $SQL -> error; // (*)

But it works out to exactly the same thing. I understand I would notice a difference only if I were looking out for SQL errors in intermediate queries sent by multi_query(), making those errors accessible with next_result() calls. Whatever -- N/A here as far as I can see.

Hilarity continues :

First off I copy-pasted a second iteration of the code above right after it, and I constate as expected that the first run through the code does not work, whereas the second one does (alternation).

The FUN thing is, that if I continue my copy-pasting until I have n copies of that code block in succession, then the first one still fails 9 times out of 10, with the same error, and then all subsequent blocks work correctly.

Next up, I decided to give up on resultsets and do other things in my multi queries ; consider the following block of code :

if (!$SQL -> multi_query('
  INSERT faq SET question = "q1", answer = "a1";
  INSERT faq SET question = "q2", answer = "a2";
  INSERT faq SET question = "q3", answer = "a3"
'))
  echo 'ERR i : #'.$SQL -> errno.' : '.$SQL -> error.'<br />'; (*)

The i on line (*) is hard-coded to correspond to the i-th block in a new copy-paste sequence. Indeed, the minimum number of times you want to copy-paste that block of code is twice, for a total a three blocks. Then :

The first block fails with the same specious concatenation problem as before :

"ERR 1 : #1064 : You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '; INSERT faq SET question = "q2", answer = "a2"; INSERT faq SET question =' at line 1"

Then the second block executes correctly, no error reported.

Then the third block manifestedly executes correctly (I do end up with TWO sets of [q1,a1 ; q2,a2 ; q3,a3] in my table), BUT multi_query() must return FALSE because I get :

"ERR 3 : #2013 : Lost connection to MySQL server during query"

And THEN, if I try to do anything else SQL-related, like for example a simple :

$SQL -> query('DELETE FROM faq WHERE question = "q1"');

Then I log a very hilarious error for that query (which does not get executed) :

"#2006 : MySQL server has gone away"

I love that.

OK, so I thought, let's look for a simple way of stopping a FIRST multi_query from failing. Well all it takes is a very simple query to "give the mysqli connection confidence" as I see it. If I add :

$SQL -> query('SELECT 1');

just after $SQL = new mysqli(...), then not only does that query not fail, but there is no longer the problem of the first multi_query failing :

- in my initial description @ 2:26pm CET, the multi_query() always works, never mind how many browser reloads.

- in this new situation with the INSERT multi_queries, the situation is TRANSLATED by 1 :

  - The first multi_query executes correctly
  - The second one executes correctly but I get the 'connection lost during query' error
  - The third one fails, because 'MySQL has gone away'.

In the place of the third multi_query, a simple query() also fails in the same manner, and so does a prepare(). $SQL -> host_info gave me correct info, however, so the $SQL object still appears to be partially valid, it just can't query the server anymore.

---------------------------------------------
---------------------------------------------

RECAP :

* A MySQLi connection appears to need to execute a simple query() before it will reliably execute multi_queries. If multi_queries are asked for immediately, the first one of them will fail.

* Whereas this problem did not appear with SELECT multi_queries, INSERT multi_queries present the following effect : the first one is fine, the second one works but makes the MySQLi object lose it's ability to further query the database server.

I hope you have as much fun solving this as I appear to be having researching it. Good luck and thank you again.

Does this problem still occur when you disable MySQL's 
query cache? 

setting query_cache_type to OFF didn't change anything. still the same error.

Please try using this CVS snapshot:

  http://snaps.php.net/php5-STABLE-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php5.0-win32-latest.zip

Produces an error in CLI with :
<?php
$SQL = new mysqli("localhost", "andrey", "", "test");

$res = $SQL->multi_query('
  SELECT indx FROM faq;
  SELECT question, answer FROM faq
');
var_dump($res, $SQL -> error, __LINE__);
do {
  $res = $SQL->store_result();
  var_dump($SQL -> error, __LINE__);
	
  if ($SQL->errno == 0) {
    while ($arr = $res->fetch_assoc())
      print_r($arr);
    $res->free();
  }
} while ($SQL->next_result());
var_dump($SQL -> error, __LINE__);

$SQL->close();
var_dump($SQL->error, __LINE__);

$SQL2 = new mysqli("localhost", "andrey", "", "test");

$res = $SQL2->multi_query('
  SELECT indx FROM faq;
  SELECT question, answer FROM faq
');
var_dump($res, $SQL2->error, __LINE__);
do {
  $res2 = $SQL2->store_result();
  var_dump($SQL2->error, __LINE__);
	
  if ($SQL2->errno == 0) {
    while ($arr = $res2->fetch_assoc())
      print_r($arr);
    $res2->free();
  }
} while ($SQL2->next_result());
var_dump($SQL2->error, __LINE__);

$SQL->close();
var_dump($SQL2->error, __LINE__);

?>

This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.

fixed :)