the same situation for //<?php

This is strange behavior of the php runtime, where the end delimiter ?> is active even in the comments. Our editor has
to reflect the runtime. See comments in http://us2.php.net/manual/en/language.basic-syntax.comments.php.

Regarding //<?php, this is treated as really php comment so it depends where it is. If it located in the php code, then
it's comment. If it is located in the html code like after //?> then it is php start delimiter.

another sample

        <?php
        $s="d";
        //<?php
        //?>
        
?>

result:
 first tag of php end  is commented but  highlighted as not commented
 second  tag of php end not highlighted  at all becouse first tag s not commented

Please, read the specification. The first tag is not commented, because php end tag even in the comment is threated as
end php tag. 

<?php    -> open php tag
$s="d";  -> a php code
//<?php  -> php comment containing open php tag, which is comment
//?>     -> php comment containing close php tag, but the close php tag is not treated as comment 
?>       -> html text, because the pho block is closed by previous php close tag

One not from the link, which I add in my previous comment:
"//?> is not treated as commented text, this is a result of having to handle code on one line such as <?php echo
'something'; //comment ?>"

Ok I read manual. 
Zend has the same behaviour.

Thank You.
I had some misunderstanding in the question.