Need a PHP function to generate a range() A to ZZZZ

I'm having trouble creating a PHP function that will output every possible letter combination, for example

a
b
c
......... etc
......... etc
........ then
aa
ab
ac
ad
......... etc
......... etc
........ then
aaa
aab
aac
......... etc
......... etc
........ then
zzx
zzy
zzz


I need the function to accept a $depth value, for example a depth of 3 would end at ZZZ.

I know the PHP range function is useful range('a','z');
but this only creates an array with A-Z

Could anyone help me with this. Thanks in advance