Hey there!

I have very long integer sequences that look like this (arbitrary length!):

0000000001110002220033333

Now I need some algorithm to convert this string into something compressed like

a9b3a3c3a2d5

Which means "a 9 times, then b 3 times, then a 3 times" and so on, where "a" stands for 0, "b" for 1, "c" for 2 and "d" for 3.

How would you do that? So far nothing suitable came to my mind, and I had no luck with google because I didn't really know what to search for. What is this kind of encoding / compression called?

PS: I am going to do the encoding with PHP, and the decoding in JavaScript.