Following code demonstrates the problem. Parser doesn't complain
about illegal (unknown) character in code. Single unknown character doesn't cause error, but two illegal characters causes error. Problem is that if that illegal character is invisible in your editor it may be very very hard to notice. If there is legal code after illegal character it's reported with "Parse error: parse error in FILENAME on line XX". Note that if parser "knows" the character - for example "!" it correctly causes parser error.

PHP build with "/configure '--with-apxs=/usr/sbin/apxs' '--with-mysql=/usr/local/mysql'".

<?php
echo("Prepare for compiler error...");
# (step 2) comment the following line to fix malicious code
?
# comment - there was error I think... parser didn't notice it!
# (step 1) uncomment following to see error
#echo("oops!");
?>
<?php
# error doesn't affect following blocks - following works OK
echo("NOT");
?>