I suggest you to use htmlspecialchars()
before sending content andhtmlspecialchars_decode()
before showing content on page, here is functions that you need to copy/paste to your functions.php :
function wp_po9568($content)
{
return htmlspecialchars($content);
}
add_filter('content_save_pre','wp_po9568');
And:
function wp_po5689($content)
{
return htmlspecialchars_decode($content);
}
add_filter( 'the_content','wp_po5689');
WordPress by default comments out php tags.