Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc.

If possible, make the script source available online and provide
an URL to it here. Try avoid embedding huge scripts into the report.

 

Sorry I misused a little.

Troubles are not in INSERTs, but in SELECTS
<?php
$mysqli = new mysqli('localhost', 'p', 'p', 'p');

$code = 1;
$language = NULL;

$stmt = $mysqli->prepare("SELECT id, val FROM table_name WHERE id = ? AND (val = ? OR ? IS NULL)");
$stmt->bind_param('iss', $code, $language, $language);
//$stmt->bind_param('i', $code);



$stmt->execute();
$stmt->bind_result($col1, $col2);

while ($stmt->fetch()) {
  printf("%s %s\n", $col1, $col2);
}

$stmt->close();
$mysqli->close();
?>

Table contains data
+----+--------+
| id | val    |
+----+--------+
|  1 | 2      |
|  1 | [NULL] |
+----+--------+

The statement above returns 0 rows.

But query
SELECT id, val FROM table_name WHERE id = 1 AND (val = NULL OR NULL IS NULL)
returns 1 row with NULL as val (as it should realy do)

AND

SELECT id, val FROM table_name WHERE id = 1 AND (val = 2 OR 2 IS NULL)
returns 1 row with 2 as val (as it should realy do)

Copy-paste sucks )))
But query
SELECT id, val FROM table_name WHERE id = 1 AND (val = NULL OR NULL IS
NULL)
returns 2 rowa with NULL as val AND 2 as val )))

It is already documented:

mysqli_prepare:
"However, markers are not allowed for identifiers (such as table or column names), in the select list that names the columns to be returned by a SELECT statement), or to specify both operands of a binary operator such as the =  equal sign."

Here placeholder is *NOT* an indentifier. Not a column name, but a variable/ Here is a rewritten example

prepare is smth like SET @BOUND_VAR = ?;
bind sets variable to NULL
$code = NULL;
$stmt->bind_param('i', $code);

SELECT id, val 
  FROM table_name 
 WHERE id = 1 
   AND (val = @BOUND_VAR OR @BOUND_VAR IS NULL)

Now  @BOUND_VAR is threated as 0 instead of NULL. that's incorrect

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.

"It's not allowed to compare marker with NULL by "? IS NULL" too."