Source
File: wp-admin/includes/meta-boxes.php
function link_categories_meta_box( $link ) {
?>
<div id="taxonomy-linkcategory" class="categorydiv">
<ul id="category-tabs" class="category-tabs">
<li class="tabs"><a href="#categories-all"><?php _e( 'All categories' ); ?></a></li>
<li class="hide-if-no-js"><a href="#categories-pop"><?php _ex( 'Most Used', 'categories' ); ?></a></li>
</ul>
<div id="categories-all" class="tabs-panel">
<ul id="categorychecklist" data-wp-lists="list:category" class="categorychecklist form-no-clear">
<?php
if ( isset( $link->link_id ) ) {
wp_link_category_checklist( $link->link_id );
} else {
wp_link_category_checklist();
}
?>
</ul>
</div>
<div id="categories-pop" class="tabs-panel" style="display: none;">
<ul id="categorychecklist-pop" class="categorychecklist form-no-clear">
<?php wp_popular_terms_checklist( 'link_category' ); ?>
</ul>
</div>
<div id="category-adder" class="wp-hidden-children">
<a id="category-add-toggle" href="#category-add" class="taxonomy-add-new"><?php _e( '+ Add New Category' ); ?></a>
<p id="link-category-add" class="wp-hidden-child">
<label class="screen-reader-text" for="newcat"><?php _e( '+ Add New Category' ); ?></label>
<input type="text" name="newcat" id="newcat" class="form-required form-input-tip" value="<?php esc_attr_e( 'New category name' ); ?>" aria-required="true" />
<input type="button" id="link-category-add-submit" data-wp-lists="add:categorychecklist:link-category-add" class="button" value="<?php esc_attr_e( 'Add' ); ?>" />
<?php wp_nonce_field( 'add-link-category', '_ajax_nonce', false ); ?>
<span id="category-ajax-response"></span>
</p>
</div>
</div>
<?php
}