How do I stop auto code changing, when switching modes?

When I switch to and from source mode, the editor changes the contents of the editor!
Normally this would be fine, but I want to be able to enter some php code in to the editor! (Yes I know it's not the best of ideas! But the editor is only available to admin users)

For example <?php print "Hello $name!"; ?> gets deleted! 

How do I stop auto code changing, when switching modes?