You have some serious syntax errors in your script there.
| <?
| ##########CHECK FOR EXISTING USERS########
| if file_exist("$user".txt)
| then header("Location: http://www.easysms.dk/fejl.php3"); }
| else mkdir (/users$user, 0755);
|
| ?>
First of all, you are quoting a variable, then leaving a literal string unquoted. Your first line should be
if file_exist($user . ".txt")
Then you have the word "then" in there. I think you mean that to be "{".
Lastly, you again have a literal string unquoted in the third line. So, what you want is:
else mkdir("/users/$user", 0755);
If you fix those errors, your problems should go away.