get value from database in string using query builder

so in controller I am getting a value from a input form in string i.e. product_name

return $request->input('product_name');

And on the behalf of this I want to get product_id of that product from database table using query builder

return category::where('product_name',$request->input('product_name'))->get('product_id');

problem is, I am getting the value in array form but i want this value in string

//output

[{"product_id":7}]

but i want it in string like

7

please help to achieve this in single line using query builder, thanks in advance

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