I found that in my script I do something like (the same tail of every log message is because of m4 macros):
xlog("L_INFO", "[MAIN] log message [$var(info)]");
$var(info) = "blablabla";
xlog("L_INFO", "[MAIN] another message [$var(info)]");
Interesting that in first xlog the $var(info) is not null !
And instead of "null" I see there something interesting values from other client's requests.
I think we should clean up all $var variables before request, shouldn't we?