wordpress源码：get_terms中exclude_tree里的$taxonomies[0]

get_terms 中 exclude_tree 里的 $taxonomies[0]？？为什么不是 $taxonomies？？
@param string|array $taxonomies Taxonomy name or list of Taxonomy names.
@type array|string $exclude Array or comma/space-separated string of term ids to exclude.
If $include is non-empty, $exclude is ignored. Default empty array.
@type array|string $exclude_tree Array or comma/space-separated string of term ids to exclude
along with all of their descendant terms. If $include is non-empty, $exclude_tree is ignored.
Default empty array.
@type int $child_of Term ID to retrieve child terms of. If multiple taxonomies are passed, $child_of is ignored. Default 0.
@type bool|int $hide_empty Whether to hide terms not assigned to any posts. Accepts
1|true or 0|false. Default 1|true.
function get_terms( $taxonomies, $args = '' )

$exclusions = array();
if ( ! empty( $exclude_tree ) ) {
$exclude_tree = wp_parse_id_list( $exclude_tree );
$excluded_children = $exclude_tree;
foreach ( $exclude_tree as $extrunk ) {
$excluded_children = array_merge(
$excluded_children,
(array) get_terms( $taxonomies[0], array( 'child_of' => intval( $extrunk ), 'fields' => 'ids', 'hide_empty' => 0 ) )
); //为什么这里不是 $taxonomies？？
}
$exclusions = array_merge( $excluded_children, $exclusions );
}

if ( ! empty( $exclude ) ) {
$exclusions = array_merge( wp_parse_id_list( $exclude ), $exclusions );
}

回复讨论(解决方案)

$taxonomy_objects = get_object_taxonomies( 'post', 'objects' );
print_r( $taxonomy_objects);

?>
will output
Array
(
[category] => stdClass Object
(
[hierarchical] => 1
[update_count_callback] =>
[rewrite] =>
[query_var] => category_name
[public] => 1
[show_ui] => 1
[show_tagcloud] => 1
[_builtin] => 1
[labels] => stdClass Object
(
...
)

...

[name] => category
[label] => Categories
)

[post_tag] => stdClass Object
(
...
)

[post_format] => stdClass Object
(
....
)

)

由上可 $taxonomies[0]

由上可看出， $taxonomies[0]　= category
我们知道，post_tag 没有　parent 属性。这种方法　是针对　! empty( $exclude_tree )　的，是对　category 而言的。
编码者直接这样调用　category ，虽没有问题，但我想，是存在　bug 的，是存在隐患的。
不知高手如何看这问题。

@param string|array $taxonomies Taxonomy name or list of Taxonomy names 分类名称或分类名称列表
由此可知 $taxonomies 是字符串或数组

不加判断的使用 $taxonomies[0] 是错误的！

版主说得对。在wordpress 手册　http://codex.wordpress.org/Function_Reference/get_terms　
所举例子中：
// no default values. using these as examples
$taxonomies = array(
'post_tag',
'my_tax',
);

$args = array(
'orderby' => 'name',
'order' => 'ASC',
'hide_empty' => true,
'exclude' => array(),
'exclude_tree' => array(),
'include' => array(),
'number' => '',
'fields' => 'all',
'slug' => '',
'parent' => '',
'hierarchical' => true,
'child_of' => 0,
'childless' => false,
'get' => '',
'name__like' => '',
'description__like' => '',
'pad_counts' => false,
'offset' => '',
'search' => '',
'cache_domain' => 'core'
);

$terms = get_terms($taxonomies, $args);
?>
$taxonomies[0]　＝　“post_tag"
----在此例中，get_terms 还能正常使用，是因为它的参数使它避免了出现问题。如果
真存在这个问题的话，还是请高用致函　wordpress　开发组吧

答案在对参数 child_of 的说明： $child_of Term ID to retrieve child terms of .If multiple taxonomies are passed, $child_of is ignored.
也就是说，数组 $taxonomies 只能有一个元 素。