This will write / overwrite a file called test.txt in the local directory of the php file. You may need to create thr file first and enable write permissions for it to work.

  $data = "START-".strftime('%c')."-START";
  $file = "test.txt"; 
  $fp = fopen($file, "w") or die("Couldn't open file for writing!");
  fwrite($fp, $data) or die("Couldn't write values to file!"); 
  fclose($fp);