Description:
------------
Requesting feature is ability to pass null parameters with ibase_prepare/execute. At this moment, a didn't found any way for selecting rows by sql statement containing param in where_clause which values can be null. 

A similar work is done, for example, in FibPlus components for delphi by autoconverting "WHERE FIELD=:param" to "WHERE FIELD IS NULL" if :param=NULL.

Reproduce code:
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CREATE TABLE TESTTABLE (
    ID         INTEGER NOT NULL,
    NULLFIELD  INTEGER
);

INSERT INTO TESTTABLE (ID, NULLFIELD) VALUES (1, NULL);
INSERT INTO TESTTABLE (ID, NULLFIELD) VALUES (2, NULL);
INSERT INTO TESTTABLE (ID, NULLFIELD) VALUES (3, 1);
INSERT INTO TESTTABLE (ID, NULLFIELD) VALUES (4, 1);

COMMIT WORK;

<?php

$db = ibase_connect('localhost:d:\inet\bases\wwwcat.gdb', 'SYSDBA', 'masterkey', 'win1251', null, 3);
$q_sql_1 = 'SELECT ID FROM TESTTABLE WHERE NULLFIELD IS NULL';
$q_sql_2 = 'SELECT ID FROM TESTTABLE WHERE NULLFIELD = 1';

$st_sql_1 ='SELECT ID FROM TESTTABLE WHERE NULLFIELD = ?';
$st_sql_2 ='SELECT ID FROM TESTTABLE WHERE NULLFIELD IS ?';
//PHPDocument1 string 10 - ibase_prepare() [<a href='function.ibase-prepare'>function.ibase-prepare</a>]: Dynamic SQL Error SQL error code = -104 Token unknown - line 1, char 45 ? 


$st1 = ibase_prepare($db, $st_sql_1);
$st2 = ibase_prepare($db, $st_sql_2);

echo $q_sql_1."\r\n";
$q1 = ibase_query($q_sql_1);
while ($r1 = ibase_fetch_object($q1)) var_dump($r1);
echo "\r\n".'-----------------------'."\r\n";

echo $q_sql_2."\r\n";
$q2 = ibase_query($q_sql_2);
while ($r2 = ibase_fetch_object($q2)) var_dump($r2);
echo "\r\n".'-----------------------'."\r\n";


echo $st_sql_1.' param = 1'."\n\r";
$q4 = ibase_execute($st1, 1);
while ($r4 = ibase_fetch_object($q4)) var_dump($r4);
echo "\r\n".'-----------------------'."\r\n";


echo $st_sql_1.' param = NULL'."\n\r";
$q3 = ibase_execute($st1, NULL);
while ($r3 = ibase_fetch_object($q3)) var_dump($r3);
echo "\r\n".'-----------------------'."\r\n";

echo $st_sql_2.' param = NULL'."\n\r";
$q5 = ibase_execute($st2, NULL);
while ($r5 = ibase_fetch_object($q5)) var_dump($r5);
echo "\r\n".'-----------------------'."\r\n";
?>

Expected result:
----------------
Expected result is ability to pass NULL params to ibase_execute and get right value as if query was WHERE NULLFIELD IS NULL


Actual result:
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Content-type: text/html

X-Powered-By: PHP/5.0.3



SELECT ID FROM TESTTABLE WHERE NULLFIELD IS NULL

object(stdClass)#1 (1) {
  ["ID"]=>
  int(1)
}
object(stdClass)#2 (1) {
  ["ID"]=>
  int(2)
}


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

SELECT ID FROM TESTTABLE WHERE NULLFIELD = 1

object(stdClass)#2 (1) {
  ["ID"]=>
  int(3)
}
object(stdClass)#1 (1) {
  ["ID"]=>
  int(4)
}


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

SELECT ID FROM TESTTABLE WHERE NULLFIELD = ? param = 1

object(stdClass)#1 (1) {
  ["ID"]=>
  int(3)
}
object(stdClass)#2 (1) {
  ["ID"]=>
  int(4)
}


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

SELECT ID FROM TESTTABLE WHERE NULLFIELD = ? param = NULL



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

SELECT ID FROM TESTTABLE WHERE NULLFIELD IS ? param = NULL



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