Additional info: If I switch my binary back to php 4.1.3, my POP3 server script works 100% inasmuch as this bug is concerned.  

Additionally, I can confirm that something is crashing in PHP in that if I dump data to a log file, the logfile ends where the data turns to garbage.  An additional log entry should be made when the message dump is complete.  This is never executed.

This is a weird problem that seems to cause PHP to hang, not core dump, and not terminate the TCP/IP connection but causes PHP to just dump garbage and just hang.

Can you isolate the line of code that causes the hang?
Use printf to output a note before each function you suspect of causing the problem; the last line output
by the script will tell you which one has hung.

We need to narrow it down to either sockets (as in ext/socket), or streams (fgets, fread, fwrite etc.),
or output buffer -- three unrelated areas.

Code I am using is:

while(ftell($mailfile) < $EndOfMessagePos)
{
 fputs($logfile, "Getting line\n");
 $line = fgets($mailfile, 4096);
 fputs($logfile, "Echoing line\n");
 echo "$line\r\n";
 $fst += strlen($line);
 fputs($logfile, "Sent $fst: $line\n");
 }
echo ".\r\n";
fputs($logfile, "Sent: CRLF\n");
fclose($logfile);

End of log file returns:
--START--
Getting line
Echoing line
Sent 11060: (unimportant email content)
Getting line
Echoing line
Sent 11147: (unimportant email content)
Getting line
Echoing line
--END--

Thus it appears to be hanging on the "echo" line.  

Again, this problem only presents itself when the script is running as a full server on a "clean" connection.  If the same exact test is run connecting via the telnet command to port 110 or by running the script from the command-line there is no problem.  It is my guess that the telnet command introduces some kind of flow control that is not present when I open a "real" POP3 connection to the same port.

It's more likely to be a problem with fgets; could you
try fflush($logfile) ing after the fputs lines, just to make sure that they're not waiting in the buffer.

Also, when php hangs, is it using a lot of CPU?

while(ftell($mailfile) < $EndOfMessagePos)

This line has the potential to cause a indefinete loop and there's no escape code in the loop, which breaks it.

ftell will return FALSE if an error occurs, which in numerical comparison resolves to 0.

Can you change that to:
while(ftell($mailfile) and ftell($mailfile) < $EndOfMessagePos)

I added requested fflush on $logfile after each fputs.  It still indicates that it is dying on the "echo".  I.e., logfile shows "Echoing line" but never reports "Sent."

Additional important correction: I'm not sure whether behavior has changed or if I was just wrong before, but when the problem presents itself the script IS shutting down and the TCP/IP connection IS disconnected.  In fact, it does so cleanly.  It executes my registered shutdown function.  In that function I had it write to the logfile and it shows that after the last "Echoing" line it DOES go through the shutdown function.  Connection_status() within the shutdown function reports "1" (aborted), but the client side definitely didn't request the abort.

I have also refined the loop so that the possible infinite loop situation is handled.  This is a welcome improvement to my code to handle an "impossible" situation (impossible because it is constrained elsewhere by logic) but it didn't have any effect on the current problem.

Does PHP eat a lot of CPU when it hangs?

Are you capable of running it under gdb and obtaining a backtrace?

make sure you are running an --enable-debug build of PHP.
gdb ./sapi/cli/php
run nameofyourscript.php
[hit CTRL-C when it hangs]
bt full

This would be ideal, because then we would know 100%
what was going on.

In light of your comments about shutdown, what happens if you comment out the fclose() line?

Also, what are your ini settings for output buffer related options? (and does changing those affect the problem?)

PHP doesn't eat a lot of cycles when it crashes.  As I corrected in a subsequent revision to the post, the connection is being closed and the script terminates--it even goes through the registered shutdown function as it terminates.  So no cycles are being eaten.

I've not used gdb.  I could, but I'm not sure if it is applicable in this case.  If I run the script from the command-line, it works fine.  If I run it by telneting into the port connected to the script, it works fine.  It only crashes if a raw (non-Telnet-based) connection is established to the script by, for example, Eudora mail client or by a test VB script that opens a raw connection without any potential telnet connection control.

As for output buffer options, I'm using the default php.ini. If you tell me what parameters specifically you'd like to know, I can look those up and/or change them.

Again, what surprises me is that it works in command-line, works when I connect to it as a daemon via Telnet, but it fails in the above-described manner when I connect to the same daemon but using a non-Telnet client; i.e., Eudora connects and fails.  If I write a custom VB app to just connect to the daemon to observe the problem, this fails as well.

Please try using this CVS snapshot:

  http://snaps.php.net/php4-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php4-win32-latest.zip

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