test.php
{code}
$example = new ArrayObject;
$example[""] = 'value';

var_dump($example);
{code}

php test.php
{result}
PHP Notice: Illegal member variable name in /path/to/script/test.php on line 11
PHP Stack trace:
PHP 1. {main}() /path/to/script/test.php:0
PHP 2. var_dump(...) /path/to/script/test.php:11

Notice: Illegal member variable name in /path/to/script/test.php on line 11

Call Stack:
0.0029 633048 1. {main}() /path/to/script/test.php:0
0.0038 634192 2. var_dump(...) /path/to/script/test.php:11

Variables in local scope (#1):
$example = ...

class ArrayObject#1 (1) {
public $ => string(5) "value"
}
{result}