Pho (more correctly written as Phở) is actually pronounced “fuh” with a rising and enquiring like tone. Think along the lines of the French word “Feu” which means fire and you are almost there.
If you pronounce it with a downward tone (written as “phò”), this actually means “Lady of ill repute” in Vietnamese so you do have to pay attention to your tone
If you pronounce it like the word Photo you need to keep trying! But don’t worry, ask your friendly waiter and you’ll it get it right soon.