can you suggest for a new cool, fun or useful project in php?!!

Hmm, this really depends on you.

Or, try to figure out some solutions to problems in every day life like companies doing stuffs like writing receipts by hand. Create a template for them.

Good.
Any more suggestion?!

a booking calendar with payment gateway maybe, 56there is a lot of call for those

A card game for example Top Trumps. A large portion of this will be javascript in order to get the 'interactivity', but this 'simple' setup helps you create suitable json data to be passed to js.
You could develop it further by hosting an online game via sockets (not for the faint-hearted), so that 2 or more people can join the same game and play against each other.
In fact I was doing this as a side project recently, when experimenting with sockets in PHP. Was a lot of fun and I learned a lot. Mainy that I wasn't as good as I thought I was!

Easy: Create physics engine which would take input from user, of current position and applied force, then use PHP drawing libraries to draw it out as easily possible so that our own grandma's can understand.
Medium: Invent encryption system which will encrypt itself at run-time. And it's password will be whatever user inputs, put through "one-way-discrete-logarhytm". But make it reversable (by having password of course).
Hard: Populate your htdocs or www with random files. Create AI which would work with AJAX which will sort them out in very simple and specific order meanwhile remembering where the origin of the file was.

These are nothing of what you may need. But if you're into programming like me (and many people over here). I recommend you going total hardcore in field you understand. All 3 are do-able and there's surely answer of Google somewhere. If you can do these 3, you're officially a good PHP programmer. Of course, it has to be working, clean and secure.