[PHP-MYSQL] How do i use SELECT* FROM (not table name but an inner joined table)?

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[PHP-MYSQL] How can i use SELECT* FROM (not table name but an inner joined table that have just been queried)

Here is my situation

On my page I have the first sql statement to inner join four tables of data together and noted that as $sql
(I have tried fetching the array and building the html table and it's all good)

I would like to add a dropdown box for the user to be able to filter the INNER JOINED table

so i have a dropdownlist which will be the $_POST['variable']

when the user select it and press submit

it will go to $input = $_POST['variable'];

now to filter it i use

$select = "SELECT * FROM [[at this point i would like to use the INNERJOINED table ]] WHERE test_id=$input ";

i don't know how to select from the just joined table at the top of my page
so anyone knows how would i do that?? thank you