I'm afraid that this is currently technically not possible, at least not so soon. We are using a third party component for the text editor and the component does not support this feature. Although we do have a source, it would take a lot of time and effort in order to implement this feature, so it is planned to be done after higher priority features and fixes have been implemented. Thank you for your understanding.
Curently you can use the proposed solutions:
1) Set background color of <? and ?> which will allow you to see where does the php block begins and ends
2) If you want it all to be colored, set background for each PHP code item
These will allow you to achieve the same usability effect. Now it maybe won't look so good but it surely
does what you need - allows to identify PHP code