PHP recursion. How to fix it

PHP recursion.
Just read the online php recursion, and asked the big God, told me that recursion is to call itself. Didn't understand. Also on Baidu saw the Hanoi tower problem. I know what happened to Hanoi, but I still don't understand recursion. Look at the cows to give a simple and understandable example ...
------Solution--------------------

function f ($n)  
{  
   $out =-1;
   if ($n <0)
       echo "Input cannot be negative";
   else if ($n ==0
------Solution--------------------
$n ==1)
       $out = 1;
   else $out =f ($n-1) * $n;  
   return $out;
}
Echo f (3);

The most classic example of factorial

