The kings of Egypt were not originally called pharaohs by the ancient Egyptians. In the late First Dynasty, the term used was nesu-bit, referring to the king, with the additional idea that the king was a combination of divine and mortal.
The term "pharaoh" for the king of Egypt developed over time, and was also used by the Hebrews and Greeks to describe the Egyptian ruler. Today, we commonly use the term "pharaoh" with reference to the kings of Ancient Egypt, including the Hyksos and Ptolemaic rulers, but usually not the Persian rulers, although many of them did have a formal Egyptian title manufactured for themselves.
In Ancient Egypt, the term "pharaoh" was not originally a royal title. Translated literally, the earliest meaning of the Egyptian word per-o was "great house", that is the palace or residence of the king and his administration. This usage is found as early as 2500 BCE. The term "pharaoh" referred to the ordinances and commands the king issued in his administration, but not to the person of the king himself. In New Kingdom times (sixteenth century BCE), it began to designate the king himself, rather like our use of "The White House" to refer to the American president or "The Crown" to refer to the British monarch.
Thus, at the beginning of the New Kingdom (Eighteenth Dynasty), we find Thutmose III (c. 1504-1450 BCE) referred to as "pharaoh." This is understood to be the earliest instance of the title being transferred to the king himself, and any reference before this period is considered an anachronism.
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The number of hieroglyphic signs gradually grew to over 7000 in total, though not all of them were used on a regular basis (as with many of the words in an English dictionary). The hieroglyphs were chosen from a wide variety of observed images, for example, people, birds, trees, or buildings. Some signs represent the sounds of the ancient Egyptian language, but indicate consonants only. No vowels were written out. Also, it was not an alphabetic system, since one sign could represent a combination of two or more consonants like the gaming-board hieroglyph which stands for the consonants "mn". Egyptologists make the sounds pronounceable by putting an "e" between the consonants, so "mn" is read as men. A vast number of other hieroglyphic signs were not pronounced at all but served to clarify meanings, such as a boat following the sound sign "dpt" which was the word for boat.