you have it backwards. you aren't looking to substitute a value for the id you've circled. that's the name of the column, it stays as it is.

it's not SELECT FROM table WHERE "userinput from dropdown selection" = :id

it's SELECT FROM table WHERE id = "userinput from dropdown selection"

the use of :id as a parameter that you are binding with ":id" => $id gets you most of the way there.

now you just need to populate $id with the value of the dropdown in the submitted form.

if you dropdown is <select name="id" I think I would expect your code to work as it is. If it doesnt I must be missing something. It's hard to tell without the form. Otherwise change to $id = $_POST['nameofyourdropdown'];