alter database backup controlfile to ['filename' | trace]
This command comes in two versions. One backs up the control file in a binary format while the other backs it up in a human readable form.
It is required if the database is running in archive log mode and a structural change was made to the database.

Backing it up in a binary format:
alter database backup controlfile to '/some/arbitrary/path';
alter database backup controlfile to '/some/arbitrary/path' reuse;

Backing it up in a human readable format:
alter database backup controlfile to trace;
alter database backup controlfile to trace as '/some/arbitrary/path';
alter database backup controlfile to trace as '/some/arbitrary/path' reuse;

SQL> alter database mount;

Database altered.

SQL> alter database backup controlfile to trace;

Database altered.

SQL> @getmytrace

TRACE_FILE
--------------------------------------------------------------------------------

D:\oracle\admin\XUE\udump\xue_ora_3308.trc
PHP code:--------------------------------------------------------------------------------
the content of getmytrace:

select a.value||b.symbol||c.instance_name||'_ora_'||d.spid||'.trc' trace_file
from
 (select value from v$parameter where name='user_dump_dest') a,
 (select substr(value,-6,1) symbol from v$parameter where name='user_dump_dest') b,
 (select instance_name from v$instance) c,
 (select spid from v$session s,v$process p,v$mystat m where s.paddr=p.addr
  and s.sid=m.sid and m.statistic#=0) d