Unable to undertstand a certain code regarding php classes

Hi,
I have a open source script and there is usage of certain code which I am unable to understand
Firstly in a given class lets say

class Testing {
    var $wacko         = "";           //Some variable value declaration
    var $trick       = 'I am going nuts';  //Some text
    var $doom         = "";           //Some variable value declaration
    var $toon         = "";           //Some variable value declaration
    var $croon         = "";           //Some variable value declaration
    var $tune         = "";           //Some variable value declaration
    var $mess        = false;           //Some variable value declaration
    var $code     ="<form action='nutsy.php' method='get'><p>
    <input type='hidden' name='abc' value='1' />
    <input type='text' name='def' value='%4\$s' size='30' />
    <input type='submit' value='go' /><br />
    <br/>
</p></form>";

Code (markup):

I am unable to understand from where the value of %4\$s
is derived in the above code.
Where would i find a tutorial for using such type of coding?
There are other instances of

%1\$s
%2\$s
%3\$s

Code (markup):

being used in the code, but I am unable to figure out how exactly they are calculated, referred or valued. This certain code of a php class has given me alot of problem
Please help, this brain wacko script is too difficult to understand.

 

There must be some method(s) of the child class(es) that uses $code value to substitute instances like %4\$s