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# XXI. Database (dbm-style) Abstraction Layer Functions

# 简介

These functions build the foundation for accessing Berkeley DB style databases.

This is a general abstraction layer for several file-based databases. As such, functionality is limited to a common subset of features supported by modern databases such as Sleepycat Software's DB2. (This is not to be confused with IBM's DB2 software, which is supported through the ODBC functions.)

# 需求

The behaviour of various aspects depends on the implementation of the underlying database. Functions such as dba_optimize() and dba_sync() will do what they promise for one database and will do nothing for others. You have to download and install supported dba-Handlers.

表格 1. List of DBA handlers

Handler | Notes
--- | ---
dbm | Dbm is the oldest (original) type of Berkeley DB style databases. You should avoid it, if possible. We do not support the compatibility functions built into DB2 and gdbm, because they are only compatible on the source code level, but cannot handle the original dbm format.
ndbm | Ndbm is a newer type and more flexible than dbm. It still has most of the arbitrary limits of dbm (therefore it is deprecated).
gdbm | Gdbm is the GNU database manager.
db2 | DB2 is Sleepycat Software's DB2. It is described as "a programmatic toolkit that provides high-performance built-in database support for both standalone and client/server applications.
db3 | DB3 is Sleepycat Software's DB3.
db4 | DB4 is Sleepycat Software's DB4. This is available since PHP 4.3.2.
cdb | Cdb is "a fast, reliable, lightweight package for creating and reading constant databases." It is from the author of qmail and can be found at http://cr.yp.to/cdb.html. Since it is constant, we support only reading operations. And since PHP 4.3.0 we support writing (not updating) through the internal cdb library.
cdb_make | Since PHP 4.3.0 we support creation (not updating) of cdb files when the bundled cdb library is used.
flatfile | This is available since PHP 4.3.0 for compatibility with the deprecated dbm extension only and should be avoided. However you may use this where files were created in this format. That happens when configure could not find any external library.

inifile

This is available since PHP 4.3.3 to be able to modify php.ini files from within PHP scripts. When working with ini files you can pass arrays of the form array(0=>group,1=>value_name) or strings of the form "[group]value_name" where group is optional. As the functions dba_firstkey() and dba_nextkey() return string representations of the key there is a new function dba_key_split() available since PHP 5 which allows to convert the string keys into array keys without loosing `FALSE`.

qdbm | This is available since PHP 5.0.0. The qdbm library can be loaded from http://qdbm.sourceforge.net.

When invoking the dba_open() or dba_popen() functions, one of the handler names must be supplied as an argument. The actually available list of handlers is displayed by invoking phpinfo() or dba_handlers().

# 安装

By using the --enable-dba=shared configuration option you can build a dynamic loadable module to enable PHP for basic support of dbm-style databases. You also have to add support for at least one of the following handlers by specifying the --with-XXXX configure switch to your PHP configure line.

警告

After configuring and compiling PHP you must execute the following test from commandline: php run-tests.php ext/dba. This shows whether your combination of handlers works. Most problematic are dbm and ndbm which conflict with many installations. The reason for this is that on several systems these libraries are part of more than one other library. The configuration test only prevents you from configuring malfaunctioning single handlers but not combinations.

表格 2. Supported DBA handlers

Handler | Configure Switch
--- | ---

dbm

To enable support for dbm add --with-dbm[=DIR].

> 注: dbm normally is a wrapper which often results in failures. This means you should only use dbm if you are sure it works and if you really need this format.

ndbm

To enable support for ndbm add --with-ndbm[=DIR].

> 注: ndbm normally is a wrapper which often results in failures. This means you should only use ndbm if you are sure it works and if you really need this format.

gdbm | To enable support for gdbm add --with-gdbm[=DIR].

db2

To enable support for db2 add --with-db2[=DIR].

> 注: db2 conflicts with db3 and db4.

db3

To enable support for db3 add --with-db3[=DIR].

> 注: db3 conflicts with db2 and db4.

db4

To enable support for db4 add --with-db4[=DIR].

> 注: db4 conflicts with db2 and db3.

> 注: This was added in PHP 4.3.2. In earlier versions of PHP you need to use --with-db3=DIR with DIR being the path to db4 library. It is not possible to use db versions starting from 4.1 with PHP prior to version 4.3.0. Also, the db libraries with versions 4.1 through 4.1.24 cannot be used in any PHP version.

cdb

To enable support for cdb add --with-cdb[=DIR].

> 注: Since PHP 4.3.0 you can omit DIR to use the bundled cdb library that adds the cdb_make handler which allows creation of cdb files and allows to access cdb files on the network using PHP's streams.

flatfile

To enable support for flatfile add --with-flatfile.

> 注: This was added in PHP 4.3.0 to add compatibility with deprecated dbm extension. Use this handler only when you cannot install one of the libraries required by the other handlers and when you cannot use bundled cdb handler.

inifile

To enable support for inifile add --with-inifile.

> 注: This was added in PHP 5.0.0 and allows to read and set microsoft style `.ini` files (like the `php.ini` file).

qdbm

To enable support for qdbm add --with-qdbm[=DIR].

> 注: qdbm conflicts with dbm and gdbm.

> 注: This was added in PHP 5.0.0. The qdbm library can be loaded from http://qdbm.sourceforge.net.

> 注: Up to PHP 4.3.0 you are able to add both db2 and db3 handler but only one of them can be used internally. That means that you cannot have both file formats. Starting with PHP 5.0.0 there is a configuration check avoid such misconfigurations.

# 运行时配置

本扩展模块在 `php.ini` 中未定义任何配置选项。

# 资源类型

The functions dba_open() and dba_popen() return a handle to the specified database file to access which is used by all other dba-function calls.

# 预定义常量

本扩展模块未定义任何常量。

# 范例

例子 1. DBA example

`<?php $id = dba_open("/tmp/test.db", "n", "db2"); if (!$id) { echo "dba_open failed\n"; exit; } dba_replace("key", "This is an example!", $id); if (dba_exists("key", $id)) { echo dba_fetch("key", $id); dba_delete("key", $id); } dba_close($id); ?>`

DBA is binary safe and does not have any arbitrary limits. However, it inherits all limits set by the underlying database implementation.

All file-based databases must provide a way of setting the file mode of a new created database, if that is possible at all. The file mode is commonly passed as the fourth argument to dba_open() or dba_popen().

You can access all entries of a database in a linear way by using the dba_firstkey() and dba_nextkey() functions. You may not change the database while traversing it.

例子 2. Traversing a database

`<?php // ...open database... $key = dba_firstkey($id); while ($key != false) { if (true) { // remember the key to perform some action later $handle_later[] = $key; } $key = dba_nextkey($id); } foreach ($handle_later as $val) { dba_delete($val, $id); } ?>`

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