Doing some testing.
A small bug it seems, line 2 should be:

$laststring = substr($mystring, strrpos($mystring, '_') + 1);

>
> $mystring = "abcd_123456_QWER_98765" ;
> $laststring = substr($mystring, strrpos($mystring, '_'));
> $numChars = strlen($laststring);
>
>