(PHP 3 >= 3.0.8, PHP 4, PHP 5)
dba_open -- Open database
( string path, string mode [, string handler [, mixed ...]] )
dba_open() establishes a database instance for
path with mode using
Commonly a regular path in your filesystem.
It 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.
Additionally you can set the database lock method with the next char.
Use l to lock the database with a .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.
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.|
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.
Returns a positive handle on success, or FALSE on failure.
It's 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 previous PHP versions, you must use semaphores to guard against
simultaneous database access for any database handler with the exception
of GDBM. See System V semaphore support.
open mode 'c' is broken for several internal handlers and truncates
the database instead of appending data to an existent database.
Also dbm and ndbm fail on mode 'c' in typical configurations (this
cannot be fixed).