First, something obviously *has* changed on the server, such as with the configuration of PHP, what is running that uses PHP, configuration files, or possibly with your alerting system so that you're now for the first time seeing an error that has been happening all along. Any unexpected and uncharacteristic behaviour on a system should also be considered as an indication of being hacked, though while possible, I suspect is unlikely in this case.
zend_new_interned_string is a function pointer that was introduced into the PHP engine with PHP 5.4.0. It is also present in PHP 5.5. The error means that the Loader has a dependency on the symbol that is not being satisfied.
This strongly indicates that for the requests that caused the error, a version of PHP prior to PHP 5.4 was used. It's not uncommon for users to have an inconsistency with PHP versions between what their web server runs and the version used for command line scripts, and that would be one thing to consider. You should also verify that your site *is* still running PHP 5.4. Correlating web request logs with the times of the errors should help you identify the precise request that was using an earlier version of PHP, and from there, investigate why.
Agree with 'phpa'.. something has indeed changed. The error occurs when the IonCube loader version does not match with the PHP version. The error indicates that your server is no longer on PHP 5.4 OR the website that generated those alerts is no longer using PHP 5.4 so it cannot load the IonCube loader 5.4.
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