How should I operate this multi-selection process?

Question

First, you can select all, or you can select some of them, but I can't figure out how to store the selected differences in the database and use the split function to split them. Please guide me. The code is as follows

Session_start(); $username = $_SESSION["username"]; echo $username; $pdo=new PDO('mysql:host=127.0.0.1;dbname=shie','root','root') ; //mysqli_query($pdo,"SET NAMES 'utf8'")%3; ; " name="item[]">Visual distortion <input type="checkbox" value="3" name="item[]">Distant vision ghosting <input type="checkbox" value=" 4" name="item[]">Visual vision fluctuation<br/> <input type="checkbox" value="5" name=&qu