Mysqdump Backup is always 0 bytes. What to do with it

Mysqdump Backup is always 0 bytes .....
PHP backup MySQL:

The first method:
"C:\Program files\mysql\mysql Server 5.5\bin\mysqldump-uroot-hlocalhost-proot aa>c:\\www\\aa\\admin\\bak\\". $_ Post[b_name];
EXEC ($MYSQLSTR); Back up is 0 bytes.
The second method:
"C:\Program files\mysql\mysql Server 5.5\bin\mysqldump-uroot-hlocalhost-proot aa>.. Bak "; Backup is still 0 bytes,
How to write the statement, I beg you.
------Solution--------------------
First, Mark, I'll look into it.
------Solution--------------------

C:\Program files\mysql\mysql Server 5.5\bin\mysqldump-u root-proot aa  > C:\www\aa\admin\bak\20130327.sql

You add a space between-u root.
------Solution--------------------
The test success code is as follows:

EXEC (' e:\xampp\mysql\bin\mysqldump-unowphp-pnowphp examples>e:\\back1253.sql ')

