file_put_contents Saves Specified Data To Specified File...

file_put_contents("file.txt", "Some Shit\r\n"); would create/overwrite file "file.txt" with data "Some shit\r\n". ("\r\n" Creates A New Line In The File)
file_put_contents("file.txt", "Some Shit\r\n", FILE_APPEND); would add to existing file "file.txt" data "Some shit\r\n".

You may need to include full path to "file.txt" for it to work correctly.

Click Here For Further File Put Contents Help