Whilst the uraeas has been an optional symbol of Ptah since his first appearance, it most commonly appears on his headress when he is in the role of royal workman-king, either as Ptah-Sokar, or later as the hybrid deity Ptah-Seker-Osiris:
During the Middle Kingdom, Ptah-Sokar (of whom I am sure you are already well aware) was further combined with the King of the Dead Osiris to become Ptah-Sokar-Osiris. In both these this forms, Ptah frequently wears the nemes headress with uraeas, or the older skull-cap with or without the uraeas present. He is also frequently shown wearing the following, more elaborate headdress:
http://www.touregypt.net/featurestories/sync5.jpg