Phew is not a word you build into a long sentence so much as a sound you write down. You say it when you let out a breath — you are hot, tired, or, most often, relieved that something is over. It can also be the little blow people make at a bad smell. It is informal. In writing it often stands alone: “Phew! That was close.” Older and younger speakers all use it; it has been on the page since the 1600s as a copy of that sharp breath out.