Question

If there are some kind of limitations for array keys in PHP ? Length ? Not acceptable strings ?

In the official documentation found only this, but there is no information about keys limitations.

A key may be either an integer or a string. If a key is the standard representation of an integer, it will be interpreted as such (i.e. “8” will be interpreted as 8, while “08” will be interpreted as “08”). Floats in key are truncated to integer. The indexed and associative array types are the same type in PHP, which can both contain integer and string indices.*

Answer 1

What is the max key size for an array in PHP?

This question is almost the exact same. But if you dont want to trust anything unofficial, just stick to using less small keys. You may even get some performance benefits out of it.

EDIT: And as the The PHP Manual says:

Note: It is no problem for a string to become very large. PHP imposes no boundary on the size of a string; the only limit is the available memory of the computer on which PHP is running..