You say that you POST the data but in your code above you are retrieving a using GET.
$PanelCode =$_GET['Law panel'];
Read about them: POST (works with headers) vs GET (it works with query strings - data after ? in url): PHP - GET & POST Methods - Tutorialspoint
Assuming you made a right POST call, following is the way to retrieve data:
$PanelCode =$_POST['Law panel'];
A sample for your reference.: Insert Data Into MySQL Using PHP
As R.Deeming shared, make sure to have parameterized query to avoid SQL Injection.