[PHP-NOTES] note 130501 added to function.unserialize

Beware: file_put_contents() is racy by design and if you use it to write serialize()d data, you may end up having error such as

    unserialize(): Extra data starting at offset 1000 of 1002 bytes

For details, see https://github.com/php/php-src/issues/20108
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Manual Page -- https://php.net/manual/en/function.unserialize.php
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