syatem
the 'a' key is directly next to the 's' key on the keyboard
seat
indeed a seat (like a chair, where people sit on)
clicking seat will change the color means is occupied how can i do this
this depends on a lot of things. if it’s just about changing the colour, you could do this with JavaScript (PHP doesn’t receive click events), given that the 'seat' has a CSS colour to begin with (if it were an image, changing CSS colours won’t help).
doing that doesn’t mean though that the seat is booked, for that you need a whole lotta code more (this time PHP and JavaScript).