如何避免重复的代码？

I've the following Problem, those methods of my SQLBuilder are simply the same, what can I do to reduce the code?

    public function select($fields){
        if(is_array($fields)){
            $this->fields = implode(',', $fields);
        } else {
            $this->fields = $fields;
        } 
        return $this;
    }

    public function from($tables){
        if(is_array($tables)){
            $this->tables = implode(',', $tables);
        } else {
            $this->tables = $tables;
        } 
        return $this;
    }

create a common private method that does the processing and then public methods that use them

public function select($fields){
    return $this->builddata($fields, 'fields');
}

public function from($tables){
    return $this->builddata($tables, 'tables');
}

private function builddata($data, $storage) {
    if(is_array($data)){
        $data = implode(',', $data);
    }
    $this->$storage = $data; // variable variable

    return $this;
}

or

public function action($data, $type){
        if(is_array($data)){
            $this->$type = implode(',', $data);
        } else {
            $this->$type = $data;
        } 
        return $this;
    }

then

$this->action($data, 'select');