Error with: passthru('mysql dbname &lt; backup.sql');

Error with: passthru('mysql dbname < backup.sql');

To load a backup file into a database I'm using the next string.

echo passthru ('c:\\mysql\\bin\\mysql -f dbname < C:\\mysql\\bin\\backup.sql');

What I get back with is exactly what you get back running "mysql --help".
Is it a problem of MySQL version or might PHP be the reason?