This signal is emitted when the
insert_text() method is called,
which happens internally every time the text is added to by the end
user, at every key press.
The string_length that is passed is the length
of the string that is being inserted, and not the length of the string
that makes up the content of the text widget.
Note that there is a fourth parameter here that the signal cannot carry
in PHP-GTK because it is a GTK_TYPE_POINTER, which is an unsupported
type. Depending on the E-NOTICES settings in your
php.ini file, you may get an error message when you
connect this signal. This does not affect the signal's functionality.
Callback function
void callback(GtkEditable widget, string text_string, int string_length);
