A string question

[Chris Payne <oxygenet32@xxxxxxxxx>]

Hi there everyone,

I basically have the following code, which grabs the 3 numbers to the right:

$input = "250705023";

$resta = substr($input, -3, 3);

Then I manipulate them a little, but my question is - how, once i've used
them to create different values - can I put them back in the original
string at the same location?

Got me a bit confused.

Chris