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XVII. Database (dbm-style) abstraction layer functions

Introduction

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.)

Requirements

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.

Table 1. List of DBA handlers

HandlerNotes
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 4.4.0.
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 here. 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 bundeled 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.

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().

Installation

By using the --enable-dba=shared configuration option you can build a dynamic loadable modul 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.

Table 2. Supported DBA handlers

HandlerConfigure Switch
dbm To enable support for dbm add --with-dbm[=DIR].
ndbm To enable support for ndbm add --with-ndbm[=DIR].
gdbm To enable support for gdbm add --with-gdbm[=DIR].
db2 To enable support for db2 add --with-db2[=DIR].

Note: db2 conflicts with db3 and db4.

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

Note: db3 conflicts with db2 and db4.

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

Note: db4 conflicts with db2 and db3.

Note: This was added in PHP 4.4.0. In earlier version you need to use --with-db3=DIR with DIR being the path to db4 librarie. It is not possible to use db versions starting from 4.1 with PHP prior to version 4.3.0.

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

Note: Since PHP 4.3.0 you can omit DIR to use the bundeled 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.

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

Note: 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 4.4.0 there is a configuration check avoid such missconfigurations.

Runtime Configuration

This extension has no configuration directives defined in php.ini.

Resource Types

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.

Predefined Constants

This extension has no constants defined.

Examples

Example 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.

Example 2. Traversing a database

<?php

// ...open database...

$key = dba_firstkey ($id);

while ($key != false) {
    if (...) {          // remember the key to perform some action later
        $handle_later[] = $key;
    }
    $key = dba_nextkey ($id);
}

for ($i = 0; $i < count($handle_later); $i++)
    dba_delete ($handle_later[$i], $id);

?>

Table of Contents
dba_close -- Close database
dba_delete -- Delete entry specified by key
dba_exists -- Check whether key exists
dba_fetch -- Fetch data specified by key
dba_firstkey -- Fetch first key
dba_handlers -- List handlers available
dba_insert -- Insert entry
dba_list -- List all open database files
dba_nextkey -- Fetch next key
dba_open -- Open database
dba_optimize -- Optimize database
dba_popen -- Open database persistently
dba_replace -- Replace or insert entry
dba_sync -- Synchronize database
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