描述：

通过页面id获取所有子页面

用法：

<?php get_page_children( $page_id, $pages ) ?>

参数：

$page_id

(integer) (必填) 页面ID.

默认值: None

$pages

(array) (必填) 页面对象列表。

默认值: None

示例：

<?php
// Set up the objects needed
$my_wp_query = new WP_Query();
$all_wp_pages = $my_wp_query->query(array('post_type' => 'page'));

// Get the page as an Object
$portfolio =  get_page_by_title('Portfolio');

// Filter through all pages and find Portfolio's children
$portfolio_children = get_page_children( $portfolio->ID, $all_wp_pages );

// echo what we get back from WP to the browser
echo '<pre>' . print_r( $portfolio_children, true ) . '</pre>';
?>

源文件：

/**
 * Identify descendants of a given page ID in a list of page objects.
 *
 * Descendants are identified from the `$pages` array passed to the function. No database queries are performed.
 *
 * @since 1.5.1
 *
 * @param int   $page_id Page ID.
 * @param array $pages   List of page objects from which descendants should be identified.
 * @return array List of page children.
 */
function get_page_children( $page_id, $pages ) {
	// Build a hash of ID -> children.
	$children = array();
	foreach ( (array) $pages as $page ) {
		$children[ intval( $page->post_parent ) ][] = $page;
	}

	$page_list = array();

	// Start the search by looking at immediate children.
	if ( isset( $children[ $page_id ] ) ) {
		// Always start at the end of the stack in order to preserve original `$pages` order.
		$to_look = array_reverse( $children[ $page_id ] );

		while ( $to_look ) {
			$p = array_pop( $to_look );
			$page_list[] = $p;
			if ( isset( $children[ $p->ID ] ) ) {
				foreach ( array_reverse( $children[ $p->ID ] ) as $child ) {
					// Append to the `$to_look` stack to descend the tree.
					$to_look[] = $child;
				}
			}
		}
	}

	return $page_list;
}