How to use array values outside foreach loop?

How can I use an array values outside a foreach loop in Laravel 5.4?
Here is the code :

    public function index(Request $request)
    {
        $name = $request->input('keyword');
        $category = $request->input('category');
        $catkeywords = array(DB::table('keywords')->pluck($category));
        foreach ($catkeywords as $catkeyword) {
            $string = implode(',',$catkeyword);
        }
        echo $string;
}

I don't know why it doesn't work !
I just want the returned keywords from database to combine them with some text and to use for some API queries.
In other words, I want the list of keywords outside of the loop.

via M. Safari