function sar () {
    from="$1"
    shift
    to="$1"
    shift

    if [[ $from == '' || $to == '' ]]
    then
        echo Usage: sar <from> <to> <filename>
        return
    fi

    while [[ $# -gt 0 ]]
    do
      filename="$1"
      shift
      find ./$filename -type f -exec sed -i -e 's/$from/$to/g' {} ;
    done
}

$ sar xxxx yyyy mydir

+ find ./mydir -type f -exec sed -i -e 's/$from/$to/g' '{}' ';'