PDO_Firebird returns null for non-null empty BLOBs

Description

Description

PDO_Firebird returns PHP null when fetching a non-NULL BLOB whose length is zero.

Expected behavior: An empty BLOB should return an empty PHP string (''), while SQL NULL should return PHP null.

Actual behavior: Both values return PHP null, although SQL expressions confirm that Firebird stores them distinctly:

• SQL NULL: IS NULL = true, OCTET_LENGTH = NULL
• Empty BLOB: IS NULL = false, OCTET_LENGTH = 0

This loses information during retrieval. Applications cannot distinguish an absent value from an explicitly stored empty value.

The behavior occurs with both text and binary BLOBs, after INSERT and UPDATE, using string or stream input. It reproduces with direct PDO, without Laravel or another framework.

Reproducer

Requires an existing Firebird database and PDO_Firebird. Set FB_DSN, FB_USER, and FB_PASSWORD as appropriate.

<?php
// Example: FB_DSN='firebird:dbname=localhost/3055:/path/database.fdb;charset=UTF8'

//          FB_USER=sysdba FB_PASSWORD=... php pdo_firebird_empty_blob.php

$pdo = new PDO(

getenv('FB_DSN') ?: 'firebird:dbname=localhost/3055:/var/lib/firebird/data/database.fdb;charset=UTF8',

getenv('FB_USER') ?: 'sysdba',

getenv('FB_PASSWORD') ?: 'masterkey',

[PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION, PDO::ATTR_ORACLE_NULLS => PDO::NULL_NATURAL]

);

$table = 'BLOB_REPRO_'.bin2hex(random_bytes(6));

$created = false;

$stmt = null;
try {

$pdo->exec("CREATE TABLE $table (ID INTEGER, PAYLOAD BLOB SUB_TYPE BINARY)");

$created = true;

$stmt = $pdo->prepare("INSERT INTO $table (ID, PAYLOAD) VALUES (?, ?)");

foreach ([null, '', "\0"] as $id => $value) {

$stmt->bindValue(1, $id, PDO::PARAM_INT);

$stmt->bindValue(2, $value, $value === null ? PDO::PARAM_NULL : PDO::PARAM_STR);

$stmt->execute();

}

$stmt = null;
$stmt = $pdo-&gt;query("SELECT ID, CASE WHEN PAYLOAD IS NULL THEN 1 ELSE 0 END,
    OCTET_LENGTH(PAYLOAD), PAYLOAD FROM $table ORDER BY ID");
while ($row = $stmt-&gt;fetch(PDO::FETCH_NUM)) {
    $value = $row[3] === null ? 'NULL' : 'string('.strlen($row[3]).') hex='.bin2hex($row[3]);
    printf("%s: IS NULL=%s; OCTET_LENGTH=%s; PHP value=%s\n",
        ['SQL NULL', 'Empty BLOB', 'NUL byte'][(int) $row[0]],
        $row[1] ? 'true' : 'false',
        $row[2] === null ? 'NULL' : $row[2],
        $value
    );
}

} finally {

// Release prepared statements before dropping the table (Firebird metadata locks).

$stmt = null;

if ($created) {

$pdo->exec("DROP TABLE $table");

}

}

Actual output

SQL NULL: IS NULL=true; OCTET_LENGTH=NULL; PHP value=NULL

Empty BLOB: IS NULL=false; OCTET_LENGTH=0; PHP value=NULL

NUL byte: IS NULL=false; OCTET_LENGTH=1; PHP value=string(1) hex=00

Expected output

SQL NULL: IS NULL=true; OCTET_LENGTH=NULL; PHP value=NULL

Empty BLOB: IS NULL=false; OCTET_LENGTH=0; PHP value=string(0) hex=

NUL byte: IS NULL=false; OCTET_LENGTH=1; PHP value=string(1) hex=00

Tested versions

Earlier framework-free PDO diagnostics reproduced the behavior with:

The exact minimal reproducer above was additionally verified with PHP 8.5.2, Firebird client 5.0.3, on macOS, against both server versions.

FETCH_ASSOC, FETCH_OBJ, and FETCH_NUM show the same behavior. Using bindColumn(..., PDO::PARAM_LOB) with FETCH_BOUND returns streams for nonempty BLOBs, but still returns null for empty BLOBs.

PDO::NULL_TO_STRING is not a solution because it also converts genuine SQL NULL values to empty strings.

Source analysis

In ext/pdo_firebird/firebird_statement.c, the relevant function is php_firebird_fetch_blob() in PHP 8.4/8.5, named firebird_fetch_blob() in PHP 8.2/8.3.

The function assigns a string to the result inside if (len). When the BLOB length is zero, that assignment is skipped, while the function still returns success.

PDO initializes the destination to NULL before invoking the driver's column-fetch callback in ext/pdo/pdo_stmt.c. This appears to explain the observed result.

PHP Version

PHP 8.5.2 (cli) (built: Jan 13 2026 21:40:53) (NTS)
Copyright (c) The PHP Group
Built by Homebrew
Zend Engine v4.5.2, Copyright (c) Zend Technologies
    with Zend OPcache v8.5.2, Copyright (c), by Zend Technologies

Operating System

macOS

Inizia in ext/pdo_firebird/firebird_statement.c, in php_firebird_fetch_blob() (o in firebird_fetch_blob() su PHP 8.2/8.3), quindi esamina l’inizializzazione della destinazione in ext/pdo/pdo_stmt.c. Esegui il riproduttore PDO fornito con Firebird e verifica che SQL NULL rimanga PHP null, mentre un BLOB vuoto diventi una stringa di lunghezza zero per tutte le modalità di fetch descritte.