大对象 (LOBs)

应用程序在某一时刻，可能需要在数据库中存储"大"数据。"大"通常意味着"大约 4kb 或以上"，尽管某些数据库在数据达到"大"之前可以轻松地处理多达 32kb 的数据。大对象本质上可能是文本或二进制。在 PDOStatement::bindParam() 或 PDOStatement::bindColumn()) 调用中使用 PDO::PARAM_LOB 类型码可以让 PDO 使用大数据类型。PDO::PARAM_LOB 告诉 PDO 作为流来映射数据，以便能使用 PHP Streams API 来操作。

Example #1 从数据库中显示一张图片

下面例子绑定一个 LOB 到 $lob 变量，然后用 fpassthru() 将其发送到浏览器。因为 LOB 代表一个流，所以类似 fgets()、fread() 以及 stream_get_contents() 这样的函数都可以用在它上面。

<?php
$db = new PDO('odbc:SAMPLE', 'db2inst1', 'ibmdb2');
$stmt = $db->prepare("select contenttype, imagedata from images where id=?");
$stmt->execute(array($_GET['id']));
$stmt->bindColumn(1, $type, PDO::PARAM_STR, 256);
$stmt->bindColumn(2, $lob, PDO::PARAM_LOB);
$stmt->fetch(PDO::FETCH_BOUND);

header("Content-Type: $type");
fpassthru($lob);
?>

Example #2 插入一张图片到数据库

下面例子打开一个文件并将文件句柄传给 PDO 来做为一个 LOB 插入。PDO尽可能地让数据库以最有效的方式获取文件内容。

<?php
$db = new PDO('odbc:SAMPLE', 'db2inst1', 'ibmdb2');
$stmt = $db->prepare("insert into images (id, contenttype, imagedata) values (?, ?, ?)");
$id = get_new_id(); // 调用某个函数来分配一个新 ID

// 假设处理一个文件上传
// 可以在 PHP 文档中找到更多的信息

$fp = fopen($_FILES['file']['tmp_name'], 'rb');

$stmt->bindParam(1, $id);
$stmt->bindParam(2, $_FILES['file']['type']);
$stmt->bindParam(3, $fp, PDO::PARAM_LOB);

$db->beginTransaction();
$stmt->execute();
$db->commit();
?>

Example #3 插入一张图片到数据库：Oracle

对于从文件插入一个 lob，Oracle略有不同。必须在事务之后进行插入，否则当执行查询时导致新近插入 LOB 将以0长度被隐式提交：

<?php
$db = new PDO('oci:', 'scott', 'tiger');
$stmt = $db->prepare("insert into images (id, contenttype, imagedata) " .
"VALUES (?, ?, EMPTY_BLOB()) RETURNING imagedata INTO ?");
$id = get_new_id(); // 调用某个函数来分配一个新 ID

// 假设处理一个文件上传
// 可以在 PHP 文档中找到更多的信息

$fp = fopen($_FILES['file']['tmp_name'], 'rb');

$stmt->bindParam(1, $id);
$stmt->bindParam(2, $_FILES['file']['type']);
$stmt->bindParam(3, $fp, PDO::PARAM_LOB);

$stmt->beginTransaction();
$stmt->execute();
$stmt->commit();
?>

User Contributed Notes

ben dot leiting at gmail dot com 14-Jun-2016 02:56
For selecting data out of Postgres, the data type of the column in the table determined if the parameter bound with PARAM_LOB returned a string or returned a resource.

<?php

// create table log ( data text ) ;
$geth = $dbh->prepare('select data from log ');
$geth->execute();
$geth->bindColumn(1, $dataString, PDO::PARAM_LOB);
$geth->fetch(PDO::FETCH_BOUND);

echo ($dataString); // $dataString is a string

// create table log ( data bytea ) ;
$geth = $dbh->prepare('select data from log');
$geth->execute();
$geth->bindColumn(1, $dataFH, PDO::PARAM_LOB);
$geth->fetch(PDO::FETCH_BOUND);

fpassthru($dataFH); // $dataFH is a resource
xorinox at gmx dot ch 13-Jan-2011 08:11
I find it easier to use stream_get_contens to fetch the data of a lob using the file handle.

<?php
$stmt = $pdo->con->prepare( 'select * from filetable' );
$stmt->execute();
$res = $stmt->fetchAll( PDO::FETCH_ASSOC );

for( $i=0; $i<count($res); $i++ ){
  $filename = "C:/tmp/".$res[$i]['FILE_ID'].'.xml';
  $content = stream_get_contents( $res[$i]['DATA_FILE'] );
  file_put_contents( $filename, $content );
}
?>
Jeremy Cook 19-Feb-2010 10:31
There seems to be a bug that affects example 1 above. PDO::PARAM_LOB when used with pdo::bindColumn() is supposed to return a stream but it returns a string. Passing this string to fpassthru() then triggers an error with the message 'supplied argument is not a valid stream resource'. This has been reported in bug #40913. The work around is to do the following:

<?php
$stmt = $db->prepare("select contenttype, imagedata from images where id=?");
$stmt->execute(array($_GET['id']));
$stmt->bindColumn(1, $type, PDO::PARAM_STR, 256);
$stmt->bindColumn(2, $lob, PDO::PARAM_LOB);
$stmt->fetch(PDO::FETCH_BOUND);

header("Content-Type: $type");
echo($lob);
?>

Since the browser is expecting an image after the call to header() writing the string representation of the binary output with echo() has the same affect as calling fpassthru().
http://matts.org/ 28-Aug-2009 10:30
A big gotcha exists for Oracle users.

You have to save CLOB objects using PDO::PARAM_STR, not PDO::PARAM_LOB.

But you MUST send the 4th argument, usually strlen($subject) or you get a LONG error.
diogoko at gmail dot com 01-Apr-2009 06:33
PDOStatement's methods bindParam and bindValue also work with strings, as in:

<?php
  $data = file_get_contents($filename);
  $stmt->bindValue(1, $data, PDO::PARAM_LOB);
  //...
?>

This was the only way I could make it work with PostgreSQL.
knl at bitflop dot com 23-Sep-2008 09:07
I spend a lot of time trying to get this to work, but no matter what I did PDO corrupted my data.

I finally discovered that I had been using:

$pdo->exec('SET CHARACTER SET utf8');

in the TRY part of my connection script.

This off course doesn't work when you feed binary input to PDO using the parameter lob.