Newbie preg_replace question

[leegold2]

What does "$0" do? The results change if I use "$1" why?

$a = "Foo moo boo tool foo";
$b = preg_replace("/[A-Za-z]oo\b/", "Got word: $0 ", $a);

Thanks,
Lee G.

 

[jpadie]

variables cannot start with a number. $0 and $1 do not refer, therefore to variables.