Blackhat SEO is a malicious technique used to manipulate the search engine results in order to benefit a website in terms of relevance. The payload is PHP based, thus intended for server-side use and the payload is executed directly on the server, while the site is loaded. Only the payload result (such as malicious iframe, or redirect) is visible in the browser, not the malicious code itself.
In this case, another obfuscation layer is added to the payload: PHP encoding.
Any vulnerable PHP based website. Outdated software or compromised passwords can act as an infection vector.
Inspect your files looking for the hidden iframes or any code that you do not recognize. Also, you can contact Sucuri to help you with the infection removal.