How to get combinations of characters in php?

Question

PHP

Let's say there are characters "123"
It is necessary to make possible combinations from all these characters, for example:
1
2
3
11
12
22
..
111
112
121
211
..
113
131
etc.

Answer 1

Iterating over possible values ​​in PHP?

Answer 2

so in a nested loop
or recursively