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
FALSE on failure.
If NDBM support is used, NDBM will actually create
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.
当安全模式被激活时，PHP 将检查被操作的文件或者目录是否与正在执行的脚本有相同的 UID（所有者）。