Description:
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Null Coalescing Operator throw a notice when key is a numeric string, just like "10-10" or "123b".

Test script:
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<?php
error_reporting(E_ALL);
date_default_timezone_set("Asia/Shanghai");

$context = ['attributes' => 'config'];

// example1
var_dump($context['attributes']['10-10'][':type'] ?? null);

// the same as example1
var_dump(isset($context['attributes']['10-10'][':type']) ? $context['attributes']['10-10'][':type'] : null);


Expected result:
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NULL

NULL

// Isset function and [Null Coalescing Operator] should not throw exceptions.

Actual result:
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Notice: A non well formed numeric value encountered in /home/zorzz/index.php on line 8
NULL

Notice: A non well formed numeric value encountered in /home/zorzz/index.php on line 9
NULL