1.  Go to Preferences -> General -> Editors -> File Associations

2.  In file types  choose .php extension

3.  In associated editors click Add button.

4.  Choose “Aptana HTML editor” from internal editors.

5.  Save your changes by clicking OK

6.  Now you should see HTML toolbar even when you have PHP file opened.

I hope this is just a temporal solution and  Aptana developers (or PDT team) will implement this feature in future realeases of PDT plugin.