function append_where(&$sql, $has_where) {
    $sql .= $has_where ? ' AND ' : ' WHERE ';
    return $sql;
}

function demo($name = '', $age = '') {
    $sql = "SELECT * FROM `employee`";
    $has_where = FALSE;
    if(!empty($name)) {
        $has_where = append_where($sql, $has_where);
        $sql .= "`name`='{$name}'";
    }
    if(!empty($age)) {
        $has_where = append_where($sql, $has_where);
        $sql .= "`age`='{$age}'";
    }
    echo $sql;
}