Questions about PHP's preg_replace function

Problems with PHP's preg_replace function

$string = "April 15, 2003";
$pattern = "/(/w+) (/d+), (/d+)/I";
$replacement = "/${1}1,/$3";
Print Preg_replace ($pattern, $replacement, $string);
/* Output
   ======
april1,2003
*/

$replacement = "/${1}1,/$3";
What does "/${1}1,/$3" mean?

$patterns = Array ("/(19|20) (/d{2})-(/d{1,2})-(/d{1,2})/",
                   "/^/s*{(/w+)}/s*=/");
$replace = Array ("//3///4///1//2", "$//1 =");
Print Preg_replace ($patterns, $replace, "{startdate} = 1999-5-27");

$replace = Array ("//3///4///1//2", "$//1 ="); What does this mean?
------Solution--------------------
In double quotes PHP tries to interpret $ as the leading of the variable
The string "${1}" will cause a variable undefined error
So I'm going to get rid of it so we can give ${1} to Prea_replace to handle it.

