PDO_ODBC truncating CHAR field Db2i driver

Description

The following code:

<?php
user = {myUser};
$password = {myPassword};
$rdb = {myDsn};
$dsn = "DSN={{$rdb}};NAM=1;ALWAYSCALCLEN=1;CCSID=1208";

$sql = "
SELECT long_char_field
 FROM table
 WHERE id = '1'
";

$attrOpions = [
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_PERSISTENT => false,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
];
$db = new PDO("odbc:$dsn", $user, $password, $attrOpions);

$stmt = $db->prepare($sql);
$stmt->execute();
while (($row = $stmt->fetch())) {
    var_dump($row["LONG_CHAR_FIELD"]);
}

$db = odbc_connect($dsn, $user, $password);

$stmt = odbc_prepare($db, $sql);
odbc_execute($stmt);
while (($row = odbc_fetch_array($stmt))) {
    var_dump($row["LONG_CHAR_FIELD"]);
}

Resulted in this output:

string(256) "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
string(560) "00000000016500000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"

But I expected this output instead:

string(560) "00000000016500000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"  
string(560) "00000000016500000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"

I've seen a lot of discussion around pdo_odbc and long field fetching, but nothing that exactly matched what I'm seeing here. Similar though. We have a field that's defined in ddl as CHAR(560) on db2 for i (using up to date drivers from here). After upgrading our php from 7.0.1 to 8.3.20 we're having the issue where this column is only returning 256 chars, and the data returned is not positionally correct (i.e. the chunks seem to get overlaid on one another).

The problematic logic seems to be the chunk approach for retrieving "long" columns starting here

I was able to debug, and patched in @NattyNarwhal 's printfs from here https://gist.github.com/NattyNarwhal/278aedf8e3c48bb8dcb4ae14cde0b283#file-debugging-printf-patch

What I see in both cases is that the driver is fetching a length of 256 at first, but the logic to continue the loop is (correctly) triggered 2x

if (rc==SQL_SUCCESS_WITH_INFO || (rc==SQL_SUCCESS && C->fetched_len > 255)) 

What I see happening as I step through is that though the original_fetched_len is 256, the status is still SQL_SUCCESS_WITH_INFO causing it to drop into this logic block that overlays the result with the new data.

				str = zend_string_realloc(str, used + 256, 0);
				memcpy(ZSTR_VAL(str) + used, buf2, 256);
				used = used + 255;

Interestingly, if we patch a rollback of this commit, eliminating the recalculation of used, things go back to working.

I also suspect that this PR would fix it too, but I don't know if that's still planned to go in.

Here's the logging output from the patch above.

!!! Get col stmt 0x7ffffe68e000 col 0
*** Is long string? 1
*** After fetching 256, fetched len 256 data "00000000016500000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
*** Actually long
@@@ fixed_used 0 = orig_fetched_len 256 - C->fetched_len 256
@@@ (and used = 255)
@@@ strange fetch_len 256, buf "000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
@@@ fixed_used 206 = orig_fetched_len 256 - C->fetched_len 50
@@@ (and used = 255)
@@@ normal fetch_len 50, buf "00000000000000000000000000000000000000000000000000"
@@@ fixed_used 256 = orig_fetched_len 256 - C->fetched_len 0
@@@ (and used = 256)
*** Unicode? 0
*** OK, got string "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"

PHP Version

PHP 8.3.20 (cli) (built: Apr 8 2025 20:21:18) (NTS gcc x86_64)
Copyright (c) The PHP Group
Zend Engine v4.3.20, Copyright (c) Zend Technologies
with Zend OPcache v8.3.20, Copyright (c), by Zend Technologies
with Xdebug v3.4.2, Copyright (c) 2002-2025, by Derick Rethans

Operating System

Red Hat Enterprise Linux release 9.5 (Plow)

Start in ext/pdo_odbc/odbc_stmt.c around the long-column retrieval logic near line 654, then compare it with commit bccca0b53aa60a62e2988c750fc73c02d109e642 and PR 10809. Reproduce the Db2 for i case with a CHAR(560) field using PHP 8.3.20 and the current drivers. Done means PDO and ODBC both return the complete 560-character value without positional overlay.