comma-formatting numbers

i'm new to php and am having difficulty translating this tiny awk function that i use to comma format numbers. can someone assist? my stumbling block thus far have been the fact the the php regex matching functions don't return the byte offset within the haystack and the regex replace functions don't support the awk and perl-like ampersand "&" in the replacement pattern. please help and thanks. function commas(num) { while (num ~ /[0-9][0-9][0-9][0-9]/) sub (/[0-9][0-9][0-9]$|[0-9][0-9][0-9][,]/, ",&", num) return num }