dba_open() establishes a database instance for path with mode using handler.
Commonly a regular path in your filesystem.
r for read access,
read/write access to an already existing database,
for read/write access and database creation if it doesn't currently exist,
n for create, truncate and read/write access.
Additionally you can set the database lock method with the next char.
l to lock the database with a
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
d. When none of
- is used, dba will lock
on the database file as it would with
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:
|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.|
|4.3.0||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.|
|4.3.0||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.|
|before 4.3.5||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).|