Generate the answer for a given question. The question requires common knowledge of English. Make sure that the answer is in detail, long, consistent with the context, and requires general knowledge of English. To be less repetitive, please vary your language for each question. Give a proper explanation to answer, and if necessary, provide the definition of various terms. And conclude your answer if it's long. If you have used other resources (e.g., Merriam Webster's.) to answer the question, include citations in your answer. You don't need to paraphrase the context of your resources, but make sure that your answer is consistent and complete.

A few days ago, a friend and I were discussing how every 'rule' of English spelling or pronunciation has an exception, and every exception has an exception as well. Then I brought up the rule of a ph cluster equaling an f sound (as in phonetic, elephant, morph, etc.) as a pronunciation rule that didn't have any exceptions I could think of. Is this a true hard-and-fast rule or does it have some exceptions as well? I'm not counting abbreviations such as pH scale.
The exceptions come in two categories: Greek words that were originally pronounced with an 'f' diphtheria, diphthong, ophthalmology, phthisis but have come to be pronounced with a 'p' by no process I understand. Compound words uphold, saphead, peephole that are just a word ending in 'p' run up against a word beginning with an 'h'. Neither of those really feel like exceptions: mispronunciations that have become accepted and two words being treated as one. Then there is aphelion. Arguably, that's a compound word and a mistake. By analogy with apogee and apastron, it should be apohelion: 'apo' ('from') + 'helion' ('sun'). Several people brought up 'Stephen', which is often pronounced like 'Steven'. Eh, I think we should play with Scrabble rules: no proper names. Finally, there is an example that will really blow your minds: phthalate. The ph- is silent.