Store Data as Json Encode in Magneto

Here we are storing a group data as json encode to database.Since Json Encode is inbuilt with magento.

We have two values that must stored as value and key .So its a array

for ($i = 0; $i < $length; $i++) {
                $productData = [
                    'form_key' => $this->formKey->getFormKey(),
                    'product' => $id,
                    'qty' => $params['qty'][$i]
                ];
                $lengtharray[$i]['length'] = $params['length'][$i];
                $lengtharray[$i]['qty'] = $params['qty'][$i];
    }
}

here we get the values stored as ‘length’ and key as ‘qty’.

this can be written with

json_encode($lengtharray) into a database field.

the values will be stored as like above shown in the length field.