How to insert comas into a number?

[eluser]daulex[/eluser]
(I need this for an old project that is not based on CI)

I have no clue how to do this.

The system: The user can type in a number, after numerous formulas and functions it changes and gets displayed on a different page, where it shows up like

10000 or 200 or 6020400 or any number for that matter, I would need numbers that are over 3 digits long to be parsed, so instead of 1000 it would show 1,000 or 1,000 and instead of for example 9284155213 it would show 9,284,155,213.

I hope you get the idea... Does anyone know any easy way to do this? (I don't really mind if it's php or javascript...)


Thanks