<?php
$a = 'Hello';
echo $a['whatever'];
?>
As some of the guys in the comments say, doing something like this is likely to lead to this error. As you can see in the above example $a, this is a string, not an array. This means that you cannot access it with the key (if you want to get the third letter in the string, it would be ok to do $a[2]).
, self::$conn $dsn , . . , , :
var_dump(self::$conn, $dsn)