Application's icon can be used for various purposes by the window managers on Windows and Unix: it can identify the application when it is minimized to the desktop, for example, or it can be displayed next to application's name in the taskbar, etc.

Unfortunately, Gtk+ 1.2.x for Win32 does not support setting the icon programmatically. But on other platforms one should do this: 

 1: <?php
 2:   // create top-level application window
 3:   $win = &new GtkWindow();
 4:   // make sure GdkWindow is created
 5:   $win->realize();
 6:   $gdkwin = $win->window;
 7:   list($pixmap, $mask) =
 8:      gdk::pixmap_create_from_xpm($gdkwin, null, "icon.xpm");
 9:   $gdkwin->set_icon(null, $pixmap, $mask);
10: ?>
The first parameter to set_icon() is a GdkWindow which can serve as an icon for the window when it is minimized. Only certain window managers support this functionality, though.