mysqldump executes but backup file is empty

mysqldump executes but backup file is empty

i try to backup mysql database with this code
$user="admin";
$pass="";
$dbname="mobiledb";
$backupFile = 'c:\\onstor.sql';
$command = "mysqldump -u ".$user." -p".$pass." ".$dbname." > ".$backupFile;
system($command);
the onstor.sql file has created but its size is 0Bytes!
whats the problem with this?