dba_open()

(PHP 4, PHP 5, PHP 7)

Open database

说明

dba_open(string $path,string $mode[,string $handler[,string$...]]): resource

dba_open() establishes a database instance for$pathwith$modeusing$handler.

参数

$path

Commonly a regular path in your filesystem.

$mode

It isrfor read access,wfor read/write access to an already existing database,cfor read/write access and database creation if it doesn't currently exist, andnfor create, truncate and read/write access. The database is created in BTree mode, other modes(like Hash or Queue)are not supported.

Additionally you can set the database lock method with the next char. Uselto lock the database with a.lckfile ordto lock the databasefile itself. It is important that all of your applications do this consistently.

If you want to test the access and do not want to wait for the lock you can addtas third character. When you are absolutely sure that you do not require database locking you can do so by using-instead oflord. When none ofd,lor-is used, dba will lock on the database file as it would withd.

Note:

There can only be one writer for one database file. When you use dba on a web server and more than one request requires write operations they can only be done one after another. Also read during write is not allowed. The dba extension uses locks to prevent this. See the following table:DBA lockingalready open$mode= "rl"$mode= "rlt"$mode= "wl"$mode= "wlt"$mode= "rd"$mode= "rdt"$mode= "wd"$mode= "wdt"not openokokokokokokokok$mode= "rl"okokwaitfalseillegalillegalillegalillegal$mode= "wl"waitfalsewaitfalseillegalillegalillegalillegal$mode= "rd"illegalillegalillegalillegalokokwaitfalse$mode= "wd"illegalillegalillegalillegalwaitfalsewaitfalse

• ok: the second call will be successfull.
• wait: the second call waits until dba_close() is called for the first.
• false: the second call returns false.
• illegal: you must not mix"l"and"d"modifiers for$modeparameter.

$handler

The name of the handler which shall be used for accessing$path. It is passed all optional parameters given to dba_open() and can act on behalf of them.

...

Optional string parameters which are passed to the driver.

Thecdb,cdb_make,flatfile,inifile,qdbmandtcadbdrivers do not support additional parameters.

Thedb1,db2,db3,db4,dbm,gdbm,lmdb, andndbmdrivers supports a single additional parameter$filemode, which has the same meaning as the$modeparameter of chmod(), and defaults to0644.

返回值

Returns a positive handle on success 或者在失败时返回FALSE.

参见

• dba_popen()Open database persistently
• dba_close()Close a DBA database

Windows does not support locking the database. You may use $_ENV to determine the OS:
$locking = (stripos($_ENV['OS'],'windows') === false ? 'd' : 'l');

As of GDBM version 1.8.3, GDBM's underlying open call uses non-blocking calls to flock() on systems that have flock(). As a result, calls with "rd" or "wd" locking modes will return error ("Can't be reader" or "Can't be writer") instead of waiting. Use "rl" or "wl" instead, to make PHP do its own locking external to GDBM.

Here's a simple example to use the dba_open function
<?php
$id = dba_open("/tmp/test.db", "n", "gdbm");
if (!$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);
?>

If you get some strange errors like
dba_open(): myDbFilename.db : Permission denied
than you are propably using PHP on a Windoze machine. You have to make sure that the following conditions are met:
1) Use an absolute path to your db file. Relative paths will cause problems with locking
2) Specify a locking mode - that's the second character of the mode-argument, or else opening a dba-file will cause several notices/warnings etc.
And a final, general note:
3) Always use the english PHP doc on this site - the translations are often old as hell and miss important informations
HTH, Nils.

Apache doesn't support Berkeley DB Btree, so you can't manipulate use db4 as the type of database if you want to do DBM authentication with Apache.
gdbm seemed to work fine though, even though it supposedly using Btree instead of hash. It makes you wonder why Apache would use hash for one dbmtype versus btree for another.
So since Apache and PHP don't have options to choose the method for the Berkeley DBs, you are out of luck.

Note the “c” create flag does not work if MySQL was built with the “cdb” DBA handler compile option which is common for many distros. By definition the cdb DBA handler is optimized for reading/writing and “no updates are allowed.” 
<?php
$dbh = dba_open( "./data2/productz", "c", "cdb") or die( "Couldn't open Database" ); 
?>
instead use
<?php
$dbh = dba_open( "./data2/productz", "n", "cdb" ) or die( "Couldnt open Database" );
?>
generates this error message in the /var/log/apache2/error.log: 
[Sun Sep 06 04:18:15 2009] [error] [client 192.168.1.125] PHP Warning: dba_open(./data2/productz,c) [<a href='function.dba-open'>function.dba-open</a>]: Driver initialization failed for handler: cdb: Update operations are not supported in /var/www/projects/testcdb-c.php on line 43
see user contributed comment under dba_handlers() to see which DBA handlers are supported by your build of MySQL and note about using “cdb” compiled DBA systems: 
also see user contributed comment under dba_replace() about incompatibilities with cdb DBA handler compiled MySQL systems.