From A to Z;Primary
P is the paternal letter just as m is the maternal letter. Look at pop, papa and pater. However, while m is associated with milk and mealtimes (mmmmm), p-words dominate infant vocabulary for the excretory processes (think about it). I don't know whether psycholinguists have yet accounted for this.
On the other hand, p is for paintings, poems, pleasure and play. For alphabet books p provides a plethora of wonderful pictures - purple penguins, pirates with pop-eyed parrots, pink poodles playing the piano - and people have played with p ever since Peter Piper picked that peck of pickled peppers.
In the 16th century a Dominican monk wrote a poem of 253 hexameter verses (Pugna Porcorum), every word of which began with a p. He was called Placentius, which may account for his affection for the letter.
P is also for phrase. In words from the Ancient Greek, ph is often used to represent the f phoneme. This applies to words directly from Greek (dolphin, apostrophe), Hebrew words thatcame to us via the Bible (pharaoh, Pharisee), and words made up from Greek roots to describe modern inventions or discoveries (photograph, phoneme). There's also the odd phoney ph - words spelled the Greek way to give them more clout. Phoney - American slang - is an example.
The Greeks also gave us the silent p in psychology and pneumonia. But the one in receipt is home-grown, due to some lexicographer mixing up French and Latin roots.