如何递归迭代字母表?

I need to write a function which iterates through alphabet (a-z) like this:
(here is an example for a-c)

a
b
c

aa
ab
ac

ba
bb
bc

ca
cb
cc

aaa
aab
aac

... and so on. (until the word has 5 characters)

知道怎么做吗?我想我需要一些递归函数.

for($char = 'a'; $char != 'aaaaaa'; $char++){
    echo $char . PHP_EOL;
}