( string path, string mode, string handler [, ...])
dba_open() establishes a database instance for
path with mode using
path is commonly a regular path in your filesystem.
mode is "r" for read access, "w" for read/write
access to an already existing database, "c" for read/write access
and database creation if it doesn't currently exist, and "n" for
create, truncate and read/write access. Additional you can set the database
lock method with the next char. Use "l" to lock the database with an .lck
file or "d" to 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 add "t" as third character. When
you are absolutely sure that you do not require database locking you can do
so by using "-" instead of "l" or "d". When none of "d", "l" or "-" is used
dba will lock on the database file as it would with "d".
handler is 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
dba_open() returns a positive handle or FALSE, in
the case the database was opened successfull or fails, respectively.
There can only be one writer for one database file. When you use dba on
a webserver 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:
表格 1. DBA locking
|already open||mode = "rl"||mode = "rlt"||mode = "wl"||mode = "wlt"||mode = "rd"||mode = "rdt"||mode = "wd"||mode = "wdt"|
|mode = "rl"||ok||ok||wait||false||illegal||illegal||illegal||illegal|
|mode = "wl"||wait||false||wait||false||illegal||illegal||illegal||illegal|
|mode = "rd"||illegal||illegal||illegal||illegal||ok||ok||wait||false|
|mode = "wd"||illegal||illegal||illegal||illegal||wait||false||wait||false|
|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 mode parameter.|
Since PHP 4.3.0 it is possible to open database files over network connection.
However in cases a socket connection will be used (as with http or ftp) the
connection will be locked instead of the resource itself. This is important to
know since in such cases locking is simply ignored on the resource and other
solutions have to be found.
Locking and the mode modifiers "l", "d", "-" and "t" were
added in PHP 4.3.0.
In PHP versions before PHP 4.3.0 you must use semaphores to guard against
simultaneous database access for any database handler with the exception of
GDBM. See System V semaphore support.
Up to PHP 4.3.5 open mode 'c' is broken for several internal handlers and
truncates the database instead of appending data to an existant database.
Also dbm and ndbm fail on mode 'c' in typical configurations (this cannot