Description:
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When converting stdClass objects to arrays (e.g. from json_decode()), they look exactly like arrays when using print_r() or var_dump() but isset() or array_key_exists() doesn't work. Then testing for keys, these functions always return false.

Test script:
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check =  new \stdClass();
$check->{"12"} = 37;
$check2 = (array) $check;


Expected result:
----------------
isset($check2[12]); -> true
isset($check2["12"]); -> true


Actual result:
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isset($check2[12]); -> false
isset($check2["12"]); -> false