Implement unlimited classification using php + mysql non-recursive method

The recursive method is the easiest way to think about using php to implement an infinite classification, but recursion occupies a lot of system resources. Therefore, we generally do not consider using recursion to implement an infinite classification using php, recursion is the easiest way to think about it, but recursion occupies system resources. Therefore, recursion is generally not considered,
This article uses non-recursive database design to implement unlimited classification
The database structure is as follows:
Id
Fid parent category ID
Class_name category name
Path classifier path, which uses id as a node to form a string similar to, 1, 2, 3, and 4.

The following data can be assumed:
Id fid class_name path
----------------------------------------------------
1 0 Category 1, 1,
2 0 Category 2, 2,
3 1 classification 1-1, 1, 3,
4 1 classification 1-2, 1, 4,
5 2 categories 2-1, 2, 5,
6 4 categories 1-2-1, 1, 4, 6,
----------------------------------------------------
PHP code:
$ SQL = "SELECT * FROM tree order by path ";
$ Result = $ nbs-> Query ($ SQL );
While ($ rows = $ nbs-> fetch_array ($ result )){
If (substr_count ($ rows ['path'], ',')> 2 ){
For ($ I = 0; $ I <(substr_count ($ rows ['path'], ',')-2); $ I ++)
Echo '';
}
Echo $ rows ['class _ name'].'
';
}
?>
$ Nbs is a database operation class.