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These explanations are the official ones from the PHP manual. Read more: http://www.php.net/manual/en/errorfunc.constants.php.
Logs
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by e-mail
The error levels also serve as values for the "Logs by e-mail" feature.
PrestaShop adds one last value, 5, which indicates that the administrator does not want to receive any notification, either for minor or major errors.
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