Why did Sina blog do this?

Why did Sina blog do this?

var $abc = 10;
function db () {
}
?>

This direct copy to the Sina blog when saved, the blog does not display, and remove the PHP tag can be, it is strange. Why does Sina do so?
Not only that, there is no echo ' in the blog

';, Halo
------Solution--------------------
Do you think if someone on your website comments, write the SQL code in the comment box to manipulate your database would you agree? If you publish a blog post, write a JS code inside a pop-up window, the page is opened not directly run your code it ~ This is for the safety of the site
