I'd say there is no error in the way PHP handles this, as you are accessing 
$obj->{'10'} by a string key ( '10' === "10" => true ). 

$obj = new stdClass;
$obj->{"10"} = 'as string';

$arr = (array) $obj;
$arr["10"] = 'as string again';

var_dump($arr);

would then produce something like

array(1) {
  ["10"]=>
  string(10) "as string again"
}

Test 1: 
--------
class Test {
    protected $25twentyFive;
}

Expected result: Parse error.
Actual result: Parse error.

Test 2:
--------
$obj = new stdClass;
$obj->{25} = 'test assign as-integer';

$arr = (array) $obj;
$arr[25] = 'test assign into array, with integer';

var_dump($arr);

Expected result 2:
-------------------
Parse error (since numeric variables are not compliant; when doing $obj->25 i 
get an parse error)

Actual result 2:
-----------------
array(2) {
  ["25"]=>
  string(22) "test assign as-integer"
  [25]=>
  string(36) "test assign into array, with integer"
}

Test 3:
--------
$obj = new stdClass;
$twentyFive = 25;
$obj->$twentyFive = 'test assing with integer';

$arr = (array) $obj;
$arr[$twentyFive] = 'test assign into array';

var_dump($arr);

Expected result 3:
-------------------
array(1) {
  [25]=>
  string(22) "test assign into array"
}

OR notice.

Actual result 3:
-----------------
array(2) {
  ["25"]=>
  string(24) "test assing with integer"
  [25]=>
  string(22) "test assign into array"
}

Proposed solution:
-------------------
When doing:
    $twentyFive = 25;
    $obj->$twentyFive = 'some string';

php should raise a notice


When doing:
    $obj->{25} = 'some string';

php should raise an error.

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

See bug #45959