Hi! I want to make Provably Fair system for the game like satoshigrid, where I have boxes with hidden prizes.

Now, I have read many info in this topic, but maybe I'm retarded, somehow I can't figure out did I put it all together in my head properly.
Please, let me know, if I am going to do this correctly:

On the server, I generate seed from the, lets say, current microtime.
Using this seed, I fill randomly positions with the prizes, using some php rand() function,
As prize amount is always the same, I don't need any user input here. No matter where they are
placed, it is always fair.

I hash that seed, and present it on my website on game launch.
After the game is complete, everyone can see, where the prizes were.
And I tell the real seed number which was hashed.

So everyone can compare it with hash, and I supply random formula I used to geneate spots (formula never changes)

So is that enough to call this approach secured and fairly proofable?