From the manual page on feof:

Warning - If passed file pointer is not valid you may get an infinite loop, because EOF fails to return TRUE.

From a script on our server:

while (1) {
  $nlines ++;
  $line = fgets($fh,1024);
  if (feof($fh)) break;

OOps! ... the file that's being analysed is our spam log, which is hard coded into the page that draws an email graph. And the slight (!) problem with the code above is that now that we have separated our email server from our web server, there is no such file.

Our new web server has been running fine ... except ... that occasionally and inexplicably, it has "maxed out" for a few minutes. And it was very hard to find (across many thousand pages!) what was happening. In the end, a top report showed me that we had one or two httpd threads grabbing all the cpu resource at 'event' time ... and a Perl script run against the log file to report only on the slowly served pages:

open (FH,"access_log");
$osid = 0;
while (<FH>) {
  $l++;
  ($h, $m, $s) = /(\d\d):(\d\d):(\d\d)/;
  $sid =$s + 60*$m + 3600 * $h;
  if ($osid) {
    $moved = $sid - $osid;
    if ($moved < -10) {
      print "$l ";
      print;
    }
  }
  $osid = $sid;
}

gave me the clue. (Isn't Perl a marvellous tool - even when helping debug PHP!)
(written 2008-09-01 02:31:00)

 H109 - PHP - Input / Output
A606 - Web Application Deployment - Apache - log files and log file tools

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Global - Tcl, PHP, Python
Think before you send
Calling procs in Tcl and how it compares to Perl
Reception
Server overloading - turns out to be feof in PHP
Injection Attacks - avoiding them in your PHP
Pointing all the web pages in a directory at a database
The Rise and Rise of First Bus Fares
Does fruit and veg drag on?
Easterholic