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<?php
add_shortcode( 'subpages', 'my_subpages' );

function my_subpages() {
	$id = get_the_ID();
	
	if ( false === $id ) {
		return '';
	}
	
	$pages = get_pages( array(
		'parent' => $id,
	) );
	
	$html = '';
	
	if ( $pages ) {
		foreach ( $pages as $page ) {
			$title = get_the_title( $page->ID );
			$href  = get_permalink( $page->ID );
			$html  .= sprintf( '<li><a href="%s">%s</a></li>', $href, $title );
		}
	}
	
	return $html ? sprintf( '<ul>%s</ul>', $html ) : '';
}