A string is an array of characters.  You have:

$arr[0] = "test";

Therefore array index 0 on that string is:

$arr[0][0] which very logically is "t"

Yes, i know that a string is an array of chars. Maybe i'm wrong,but i remember once only the syntax $string{0} was working, non $string[0] also.

Anyway, why this works?
php -r '$arr = 'test';var_dump($arr['rasmus']);'
still prints "t".

There is no key called "rasmus" in the $arr string...

Because non-integer keys are converted to integers. Everything is explained here: 
http://www.php.net/manual/en/language.types.string.php

Yes, you are righ, i have found that.
"Warning
[...] Non-integer types are converted to integer. [...]"

However i find this behavior a bit strange. If the string is an array, it should behave as an array, and generate an error on a non existend key.
Otherwise, if do not know if $arr is a string or array, i have to use 3 functions for checking the existence of a value: is_array($arr) && array_key_exists('smth', $arr) && isset($arr['smth']), when i could simply use isset($arr['smth']) if things worked as expected.

But ok, nevermind. A documented bug is a feature :)
Sorry for opening this thread.