Description:
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When I try to decode a json string that include a key with identifiers,then it will be converted to an invalid propery name.

Test script:
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<?php
$string = '{"test-me+for": "X"}';
var_dump(json_decode($string)); 

Expected result:
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Fatal error: syntax error, unexpected ...

Actual result:
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object(stdClass)#1 (1) { ["test-me+for"]=> string(1) "X" }