properly not a Object Oriented Programming vision, but a container-wrapper vision:

layer_manager:
+ app_window
  + gtkWindow
    + gtkfixed

in that gtkfixed, i place all the widgets.

• a vbox that includes 2 (or 3) areas:
• canvas (the place where plugins put their objects)
• canvas_bg
• canvas_bar (obviously, the bar on bottom of window)

the map that links gnome items to php-gtk items is this:

• pixmap: an extension of GtkButton (with a pixmap into it)..
• group: gtkfixed
• button: gtkbutton
• text: gtktext (but i think that is a label... i don't know).
• vbox: gtkvbox (but not used).
• every gnome item is "embedded" into a class, CanvasItem.

in the constructor of these items, the original functions have variable arguments; i used an hash as the third argument (with the key->value data as x=>100, y=>200). the pixmaps used into the canvasitem are returned from a gdk_pixbuf_new_from_filename that returns 3 elements: picture, mask, and the dimensions (loaded from the 4th line of xpm file, that contains width and height).

i used domxml for reading xml data, loading completely the tree in memory. i didn't reach to use the xpath section (for specify queries into xml), things should be more easy. gnome uses xml files for preferences so you can found gnome.xml in this directory. (but it's not used totally).

the files and naming of plugins: all plugins are into the directory specified into gcompris_main.php plugins are classes. if i have a plugin named "pippo", the classname is pippo_plugin and it is stored in a file (in the path of plugins) named pippo.php the initialization of plugins is made by the function init on every classes. the only argument is a reference to BoardPlugin, in the file plugins.php; it is used to call all the functions of the plugin (if configured) from the "plugin manager". in c this is done by creating a struct with variable functions (callbacks).