How do i add . to a string?

Hi, i'm having a pricelist and a customer cart on my website and im woundering how i can add dots for the price when the length gets 4 , 5, 6, and so on
for example the current display for one thousand is: 1000 and i want it to be 1.000 and if it's 80000 i want it to be 80.000 and so on
anyone know a simple way to do this? =) please share
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