Question: I was wondering how I could get a HTML/PHP file to read the URL,

What I mean is, Let's say I had a Search function in (index.php) that had a button saying Show Bans which opened a New Windows but when clicking Show Bans it sent a link to 'banaccount.php?id=(idhere) how can I get the PHP file (Banaccount.php) to recognise the ID and have it as a PHP function ($id = (idhere) );?

Answer: 
As D.E.Knuth's has amply documented in his vol 2 of "The Art of Computer Programming" implementing arithmetic with arbitrary precision isn't "black art" - see the chapter "Seminumerical Algorithms". I've been through that, following the necessities of implementing COBOL computationals (so: not only integers).

Also, making a workalike of BigInteger or BigDecimal without recreating an object for each new op result is an option. Yes, it means work, but if you really think you need it...

I have found that BigInteger isn't so slow as far as the arithmetic is concerned. What really killed using that for me was the necessity to convert between binary and decimal (array of decimal digits) frequently.
Your options are:

`BigDecimal` - Accurate and effective - slower than primitives but probably no measurably so - mostly immutable (i.e. can change accuracy but not value).

`long` - Accurate but has value limits - primitive so cannot be bested for speed - immutable - maths is easier and clearer to write.

`BigInteger` - Probably your best half-way house between the above - immutable so you have to make new ones whenever you change the value - you are unlikely to hit it's limits.