<? 
  $file = fopen("C:\Test.Txt", "w");
  fputs($file, "0123456789\r\n");
  fputs($file, "0123456789\r\n");
  fclose($file);
  
  $file = fopen("C:\Test.Txt", "r+");
  $line = fgets($file, 1024);
 
  echo ftell($file)."<P>";  
  echo strlen($line)."<P>";  

  fseek($file, ftell($file) - strlen($line));
  fputs($file, "test");
  fclose($file);

  echo "Done";
?>

The code above produces different outputs depending on if php.exe or php4isapi.dll is run.  Both create a file with 2 lines with 0-9 on a line.  The php.exe will then back up and overwrite the 0-3, but the php4isapi.dll will back up and overwrite the 1-4.

Output of pgp.exe
=================
12<P>12<P>Done

Output of php4isapi.dll
=======================
13<P>12<P>Done

This bug has been reproduced on Windows 95/98/NT/2000 and on version 4.0, 4.0.1, 4.0.1pl1, 4.0.1pl2.