Writing to a file

I wonder if someone would tell me the command in PHP that allows me to write to a file --- for example "test.txt" Just for this example I would like the word "apple" to be the contents of "test.txt" Now if I want to re-create "test.txt" --- erasing all the contents and start over and this time have the word "orange" in it how would you do this? What commands do you use? Thanks for your help. Ron