To add a slightly different point of view to many nice answers and comments here: Probably, there is no correct answer to this question, and it is not specific to Russian only. For example, the American abbreviation SPIE is pronounced both as S-P-I-E and as [spai] by native speakers.
In Russian, not only the traditions but also the knowledge of foreign languages define how a person will pronounce a particular word. My IT professor in university tended to pronounce FALSE as [фальш], the reason being her bad knowledge of English and a logical use of the corresponding German word.
Accordingly, pronunciation such as [пэ-ха-пэ] for PHP come from Latin, since in Russian schools Latin letter names are used: [цэ] for c, [икс] for x, [ха] for h etc.
That said, I have noticed that those with good knowledge of English tend to pronounce the abbreviations, mostly coming from English, in the proper English way: using [дабл-ю] for w, [эйч] for h etc. Simply because they use the same conventions when e.g. spelling words in English.
Of course, traditions play an important role here. While the language name С will be in most cases pronounced as [си], it will also often be declined:
писать на сях.
Finally, some IT terms have equivalents in Russian; in these rare cases Russian words are often used: [ада] for Ada, [опал] for Opal, [питон] for Python etc.