基于主键从mysqli查询创建JSON

I am probably phrasing this poorly, but I am pulling data from a database and I want to turn it into a JSON file.

My problem is I want to save it into JSON with this structure:

{
    "PRIMARYKEY": { 
        "COLUMN1":"VALUE1",
        "COLUMN2":"VALUE2"
    }
}

And it's tripping me up. I'm getting a message about not being able to use object of type mysqli_result as an array, but I am using fetch_assoc(). How do I convert this mysqli_result to a more useable format and get it into the JSON format I need? I want it in this JSON format because it seems like the best way to handle pulling information and searching the JSON file.

I also figured out my own question I foreached my query and did the following:

$JSONVARIABLE[$row["PrimaryKey"]] = array(
'Column1' => $row['Column1'], 
'Column2' => $spell['Column2']
);

I don't know why this gave me so much trouble, it was really obvious in the end. Still new at this I guess. Hopefully this will help someone else later on. Creates the JSON I need in the proper format. Thanks for the help everyone!