The 'OO' in that context stands for 'object-oriented'.
However, we haven't gone through the details of object-oriented language in PHP.
Object-oriented programming/scripting/coding (be it PHP or any other language) is a particular style of coding, and is not a separate language in itself. 'English' and 'Spanish' obviously are distinct languages. We therefore use the definite article when referring to the language that is English, as we're referring to a discrete entity. So, as per your example:
However, we haven't gone through the details of the English language in business settings.
We would not use a definite article when talking about a particular style or usage of English, and would instead say:
However, we haven't gone through the details of business language in English.
Just like object-oriented PHP, 'business language' in English is nebulous and indefinite — it is not a specific language. Hence, no definite article.