//生成随机用户名
function get_username()
{
    $chars1 = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    $chars2 = "abcdefghijklmnopqrstuvwxyz0123456789";
    $username = "";
    for ( $i = 0; $i < mt_rand(2,3); $i++ )
    {
        $username .= $chars1[mt_rand(0,25)];
    }
    $username .='_';

    for ( $i = 0; $i < mt_rand(4,6); $i++ )
    {
        $username .= $chars2[mt_rand(0,35)];
    }
    return $username;
}