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Instead of "parse error" I often get an Internal Server Error 500, which says nothing about what was wrong.

I tried to get closer to focus the problem, so I made simply file... but it is crashing even without any php in it:
-----------------------
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
	<title>title</title>
</head>

<body>
this is normal file
</body>
</html>
--------------------------------
It's addres is http://wio4.zsi.pl/del_it.html
Other files (with PHP without errors) works fine, working with MySQL.

Problems started after installing PHP 4.2.1

Finally, I returned to PHP 4.1.2 and got number of line with parse error. This time it was:

if (isset($_POST["usermail"]) and isset($_POST["userpwd"]) and !isset($_SESSION["id"]==0)) {

(last part, after !isset was wrong)
Version 4.1.2 reports parse error while 4.2.1 causes error 500.

Are you using the CGI executable or the Apache module?
How did you configure either of them in your httpd.conf?

I'm using PHP as Apache module. It is configured in httpd.conf. Works fine with PHP 4.1.2

Overwriting PHP with 4.1.2 solved all mentioned problems, so I thing that problem source is somewhere inside of PHP 4.2.1

My guess is that when you updated to 4.2.1 you didn't 
replace the correct files (and ALL of them) with new ones.

For example php4ts.dll HAS to be replaced. Also make sure you don't have any duplicates of any of the dlls in your
system.

I have cleared whole folder C:\PHP and downloaded archives of 4.1.2 (PHP stopped working) and then installed PHP 4.2.1

Yes, but are you sure there aren't duplicate php4ts.dll's anyplace else in your system? Did you search for it?

I searched everywhere for any files containing "php" in it's name. They were only in C:\PHP (deleted) and in PHP archives (zip) in download folder (I removed them also).

Your sugestion could have sense anyway, becouse phpinfo() was still showing v. 4.1.2 - that's why I was looking ror old PHP remains, but found nothing. I said to myself "somebody forgot to change it"... well, I could be wrong. I don't want to format HDD to install new PHP. Now I'm happy using old one.

If phpinfo() still shows the old version after an update,
you definately have some old dll getting loaded instead
of the new one, and that's no bug..