Hi Marigold, welcome to the Southwest!
I'm not from Phoenix, but I am in Arizona. You certainly could plant Seeds now, but I think the extreme August heat will just be too much on your seedlings. The tender babies will really struggle.
With that said, I think you will be more successful if you wait for the extreme heat to subside. No need to go through the work and excitement of a new garden during triple digit temperatures. Mature plants have a tough time during our summer heat plus many veggies won't set flowers/fruit in such heat either.
Instead, look at the Vegetable planting calendar available on this site under Tools & Apps. I use it as my guide and it sure seems to work out for me. I think you will have a thriving fall/winter garden in no time.
Keep us updated!
Happy Gardening!
