The first argument is the full-path filename of the DBM file to be opened and the second is the file open mode which is one of "r", "n", "c" or "w" for read-only, new (implies read-write, and most likely will truncate an already-existing database of the same name), create (implies read-write, and will not truncate an already-existing database of the same name) and read-write respectively.
Returns an identifier to be passed to the other DBM functions on success, or FALSE on failure.
If NDBM support is used, NDBM will actually create filename.dir and filename.pag files. GDBM only uses one file, as does the internal flat-file support, and Berkeley DB creates a filename.db file. Note that PHP does its own file locking in addition to any file locking that may be done by the DBM library itself. PHP does not delete the .lck files it creates. It uses these files simply as fixed inodes on which to do the file locking. For more information on DBM files, see your Unix man pages, or obtain GNU's GDBM.
注: 当 安全模式（safe mode） 被激活，PHP 将检查您将要操作的文件或者目录与正在执行的脚本是否有相同的 UID。