In my D7 template php I preprocessed some node variables as follows. It doesn't seem to be working in Backdrop. I went through the manual, but couldn't find an answer—is there a change in the way this should be structured? I actually have several of these, but this is the basic structure.
function myTemplate_preprocess_page( & $variables, $hook ) {
$myimg;
if ( $variables[ 'is_front' ] != TRUE ) {
if ( isset( $variables[ 'node' ]->field_image[ 'und' ][ 0 ][ 'uri' ] ) ) {
$myimg = file_create_url( $variables[ 'node' ]->field_image[ 'und' ][ 0 ][ 'uri' ] );
}
}
}
All I end up with is my website url without the complete path to the image.
OK, obviously there is a problem, but it's really hard to pinpoint with the information you have provided. Check to be sure you are not duplicating the function name. Many themes provide implementations of template_preprocess_node, so it's easy to overlook this and duplicate the function name.
Have you check the php error log? How about the Backdrop log?
template_preprocess_node() is a commonly used hook, and there is no reason why it should fail, unless you are doing something wrong.