The approach is rather interesting BUT…

It could be all right… But suddenly I need to transfer element 7 with (or without) saving the entire structure under, for instance, element 5. I will have to lots of corrections!!! And what if there are lots elements within structure of 7?

Of course, I mean that case only when sorting is important and it can be managed.

Left indent is done taking into account level field.

What for? It’s quite possible to calculate the indent with formula length(sortorder)/2 (for this case).
I.e. level field is likely to be unnecessary.
To reduce sortorder field you may convert each number added to the string into another calculation system (for example, with base of 128 instead of the one with 10) – and the string is going to reduce a bit.

Some time ago I used what had been done in forum but because of the impossibility to choose a branch without having examined the whole tree I crossed to the model used on frashmeat, sourceforge, gforge and so on. Of course, administration scripts are more complicated – they should be placed on cron but in general everything is much easier and more convenient. At this one element can be placed into some categories at the same time.

Everything is OK. The only thing which lacks is $max variable calculation. It’s a pity. The author should simply have forgotten to copy this string. And without it str_pad() doesn’t work. And following sortorder won’t have desirable zeroes.