[PHP] Simple fwrite question

[Hartley, Matt]

I am trying to have a counter in a file.  With what I have below I get in
the $counterfile

0 + 01 = 01
01 + 01 = 012
012 + 01 = 01213

[snip]

  /* Add "1" to the counter */
    $counter = ($counter + '01');
    
    /* Verify counter file is writable */
    if (is_writable($counterfile)); /* { */
        if (!$handle = fopen($counterfile, 'a')) { 
          print "Cannot open file ($counterfile)";
          exit;
    } 
    /* Write to our counter file. */
      if (!fwrite($handle, $counter)) {
          print "Cannot write to file ($counterfile)";
          exit;
    }
    fclose($handle); 

[/snip]

How do I overwrite the existing value with my new value instead of it adding
on after the past values?

My other dilemma is, 

this counter decides what line to display in a sequence from a separate file
what command can I use to tell it when $counter is @ the last line in my
"tips" file to reset to zero

thanks
Matt