PHP 7 - Arrays | Undefined index --> Bug or works as designed? - php-7

Question

I was just testing PHP 7 because of the speed improvements and now I have following code which leads to unpredictable results:
<?php
$_SESSION ['test'] [ 't****' ] = 'Hello';
var_dump($_SESSION);
echo "\n" . $_SESSION [ 'test' ] [ 't****' ];
$var = "test2";
for ($i = 4 ; $i > 0 ; $i--){
$var [$i] = "*";
echo "\n" . $_SESSION['test'][$var];
}
?>
In this example I expect 3 times an undefined index error. The last index should be found and return Hello. This works in PHP 5.6, but not in PHP 7.
In PHP 7 I get following results:
array(1) {
["test"]=>
array(1) {
["t****"]=>
string(5) "Hello"
}
}
Hello
Notice: Undefined index: test* in /in/YANSb on line 12
Notice: Undefined index: tes** in /in/YANSb on line 12
Notice: Undefined index: te*** in /in/YANSb on line 12
Notice: Undefined index: t**** in /in/YANSb on line 12
In PHP 5.6 I get the expected result:
array(1) {
["test"]=>
array(1) {
["t****"]=>
string(5) "Hello"
}
}
Hello
Notice: Undefined index: test* in /in/YANSb on line 12
Notice: Undefined index: tes** in /in/YANSb on line 12
Notice: Undefined index: te*** in /in/YANSb on line 12
Hello
I am able to reproduce the error.
The strange thing is that $var === "t****" returns true when $i == 1, but $_SESSION ['test'] [$var] will return an index undefined error although $_SESSION ['test'] ['t****'] will return Hello. So, was I on a bug hunt today?