Importing PHP code fails

I have some posts with PHP code embedded which need to be imported. My site uses a plugin which enables PHP code execution in the body text of a post. After import, the PHP code is mangled and can not be executed. Example:

<?PHP
echo "Hello world";
?>

becomes

<p><?php<br />
echo "Hello world";<br />
?></p>

Any thoughts how to avoid the mangling?

BTW, yes, I’m doubling the double quotes in the actual imported .csv file, the issue I’m having is not related to that, importing seems to be specifically mangling only the PHP portions of the body. Everything else imports perfectly.