diff --git a/tests/phpunit/tests/admin/includes/bookmark/addLink.php b/tests/phpunit/tests/admin/includes/bookmark/addLink.php
new file mode 100644
index 0000000000000..f9f7cbce1aa5e
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/bookmark/addLink.php
@@ -0,0 +1,38 @@
+<?php
+
+/**
+ * Test the `add_link()` function.
+ *
+ * @group bookmark
+ * @covers ::add_link
+ */
+class Tests_Admin_Includes_Bookmark_addLink extends WP_UnitTestCase {
+
+	/**
+	 * Tests that add_link() inserts a link from the submitted values.
+	 *
+	 * @ticket 66019
+	 */
+	public function test_add_link_inserts_submitted_link() {
+		$user_id = self::factory()->user->create( array( 'role' => 'administrator' ) );
+		update_option( 'link_manager_enabled', 1 );
+		wp_set_current_user( $user_id );
+		$_POST = array(
+			'link_name'  => 'Example link',
+			'link_url'   => 'https://example.com',
+			'link_image' => '',
+			'link_rss'   => '',
+		);
+
+		$link_id = add_link();
+
+		$this->assertIsInt( $link_id );
+		$this->assertSame( 'Example link', get_bookmark( $link_id )->link_name );
+	}
+
+	public function tear_down() {
+		$_POST = array();
+		wp_set_current_user( 0 );
+		parent::tear_down();
+	}
+}
diff --git a/tests/phpunit/tests/admin/includes/bookmark/editLink.php b/tests/phpunit/tests/admin/includes/bookmark/editLink.php
new file mode 100644
index 0000000000000..fed8f62e25b9b
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/bookmark/editLink.php
@@ -0,0 +1,43 @@
+<?php
+
+/**
+ * Test the `edit_link()` function.
+ *
+ * @group bookmark
+ * @covers ::edit_link
+ */
+class Tests_Admin_Includes_Bookmark_editLink extends WP_UnitTestCase {
+
+	/**
+	 * Tests that edit_link() updates an existing link from the submitted values.
+	 *
+	 * @ticket 66019
+	 */
+	public function test_edit_link_updates_existing_link() {
+		$user_id = self::factory()->user->create( array( 'role' => 'administrator' ) );
+		update_option( 'link_manager_enabled', 1 );
+		wp_set_current_user( $user_id );
+		$link_id = self::factory()->bookmark->create(
+			array(
+				'link_name' => 'Original name',
+				'link_url'  => 'https://old.example.com',
+			)
+		);
+		$_POST   = array(
+			'link_name'  => 'Updated name',
+			'link_url'   => 'https://new.example.com',
+			'link_image' => '',
+			'link_rss'   => '',
+		);
+
+		$this->assertSame( $link_id, edit_link( $link_id ) );
+		$this->assertSame( 'Updated name', get_bookmark( $link_id )->link_name );
+		$this->assertSame( 'https://new.example.com', get_bookmark( $link_id )->link_url );
+	}
+
+	public function tear_down() {
+		$_POST = array();
+		wp_set_current_user( 0 );
+		parent::tear_down();
+	}
+}
diff --git a/tests/phpunit/tests/admin/includes/bookmark/getDefaultLinkToEdit.php b/tests/phpunit/tests/admin/includes/bookmark/getDefaultLinkToEdit.php
new file mode 100644
index 0000000000000..9c1447cb06ddd
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/bookmark/getDefaultLinkToEdit.php
@@ -0,0 +1,62 @@
+<?php
+
+/**
+ * Test the `get_default_link_to_edit()` function.
+ *
+ * @group bookmark
+ * @covers ::get_default_link_to_edit
+ */
+class Tests_Admin_Includes_Bookmark_getDefaultLinkToEdit extends WP_UnitTestCase {
+
+	/**
+	 * Tests that get_default_link_to_edit() builds the expected default object.
+	 *
+	 * @ticket 66019
+	 * @dataProvider data_default_link_values
+	 *
+	 * @param array  $get          Query-string values.
+	 * @param string $expected_url  Expected link URL.
+	 * @param string $expected_name Expected link name.
+	 */
+	public function test_get_default_link_to_edit( $get, $expected_url, $expected_name ) {
+		$_GET = $get;
+
+		$link = get_default_link_to_edit();
+
+		$this->assertSame( $expected_url, $link->link_url );
+		$this->assertSame( $expected_name, $link->link_name );
+		$this->assertSame( 'Y', $link->link_visible );
+	}
+
+	/**
+	 * Data provider for test_get_default_link_to_edit().
+	 *
+	 * @return array<string, array{
+	 *     get: array<string, string>,
+	 *     expected_url: string,
+	 *     expected_name: string,
+	 * }>
+	 */
+	public function data_default_link_values(): array {
+		return array(
+			'empty values'    => array(
+				'get'           => array(),
+				'expected_url'  => '',
+				'expected_name' => '',
+			),
+			'provided values' => array(
+				'get'           => array(
+					'linkurl' => 'https://example.com/?a=1&amp;b=2',
+					'name'    => 'Example &amp; name',
+				),
+				'expected_url'  => 'https://example.com/?a=1&#038;b=2',
+				'expected_name' => 'Example &amp; name',
+			),
+		);
+	}
+
+	public function tear_down() {
+		$_GET = array();
+		parent::tear_down();
+	}
+}
diff --git a/tests/phpunit/tests/admin/includes/bookmark/getLinkToEdit.php b/tests/phpunit/tests/admin/includes/bookmark/getLinkToEdit.php
new file mode 100644
index 0000000000000..23e837ae8a9eb
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/bookmark/getLinkToEdit.php
@@ -0,0 +1,30 @@
+<?php
+
+/**
+ * Test the `get_link_to_edit()` function.
+ *
+ * @group bookmark
+ * @covers ::get_link_to_edit
+ */
+class Tests_Admin_Includes_Bookmark_getLinkToEdit extends WP_UnitTestCase {
+
+	/**
+	 * Tests that get_link_to_edit() returns the link in edit context.
+	 *
+	 * @ticket 66019
+	 */
+	public function test_get_link_to_edit_returns_link_for_editing() {
+		$link_id = self::factory()->bookmark->create(
+			array(
+				'link_name' => 'A link',
+				'link_url'  => 'https://example.com',
+			)
+		);
+
+		$link = get_link_to_edit( $link_id );
+
+		$this->assertInstanceOf( stdClass::class, $link );
+		$this->assertSame( $link_id, $link->link_id );
+		$this->assertSame( 'A link', $link->link_name );
+	}
+}
diff --git a/tests/phpunit/tests/admin/includes/bookmark/wpDeleteLink.php b/tests/phpunit/tests/admin/includes/bookmark/wpDeleteLink.php
new file mode 100644
index 0000000000000..1eece47921d1d
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/bookmark/wpDeleteLink.php
@@ -0,0 +1,44 @@
+<?php
+
+/**
+ * Test the `wp_delete_link()` function.
+ *
+ * @group bookmark
+ * @covers ::wp_delete_link
+ */
+class Tests_Admin_Includes_Bookmark_wpDeleteLink extends WP_UnitTestCase {
+
+	/**
+	 * Tests that wp_delete_link() removes the link and its category relationships.
+	 *
+	 * @ticket 66019
+	 */
+	public function test_wp_delete_link_removes_link_and_categories() {
+		$link_id     = self::factory()->bookmark->create();
+		$category_id = self::factory()->term->create( array( 'taxonomy' => 'link_category' ) );
+		wp_set_object_terms( $link_id, $category_id, 'link_category' );
+		add_action( 'delete_link', array( $this, 'record_deleted_link' ) );
+		add_action( 'deleted_link', array( $this, 'record_deleted_link' ) );
+
+		$this->assertTrue( wp_delete_link( $link_id ) );
+		$this->assertNull( get_bookmark( $link_id ) );
+		$this->assertSame( array(), wp_get_link_cats( $link_id ) );
+		$this->assertSame( array( $link_id, $link_id ), $this->deleted_link_ids );
+	}
+
+	/**
+	 * Records a link ID passed to a deletion action.
+	 *
+	 * @param int $link_id Deleted link ID.
+	 */
+	public function record_deleted_link( $link_id ) {
+		$this->deleted_link_ids[] = $link_id;
+	}
+
+	/**
+	 * Link IDs recorded by the deletion hooks.
+	 *
+	 * @var int[]
+	 */
+	private $deleted_link_ids = array();
+}
diff --git a/tests/phpunit/tests/admin/includes/bookmark/wpGetLinkCats.php b/tests/phpunit/tests/admin/includes/bookmark/wpGetLinkCats.php
new file mode 100644
index 0000000000000..9e41af12edc29
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/bookmark/wpGetLinkCats.php
@@ -0,0 +1,23 @@
+<?php
+
+/**
+ * Test the `wp_get_link_cats()` function.
+ *
+ * @group bookmark
+ * @covers ::wp_get_link_cats
+ */
+class Tests_Admin_Includes_Bookmark_wpGetLinkCats extends WP_UnitTestCase {
+
+	/**
+	 * Tests that wp_get_link_cats() returns the IDs of a link's categories.
+	 *
+	 * @ticket 66019
+	 */
+	public function test_wp_get_link_cats_returns_category_ids() {
+		$link_id      = self::factory()->bookmark->create();
+		$category_ids = self::factory()->term->create_many( 2, array( 'taxonomy' => 'link_category' ) );
+		wp_set_object_terms( $link_id, $category_ids, 'link_category' );
+
+		$this->assertSameSets( $category_ids, wp_get_link_cats( $link_id ) );
+	}
+}
diff --git a/tests/phpunit/tests/admin/includes/bookmark/wpInsertLink.php b/tests/phpunit/tests/admin/includes/bookmark/wpInsertLink.php
new file mode 100644
index 0000000000000..ef693f07ca6af
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/bookmark/wpInsertLink.php
@@ -0,0 +1,61 @@
+<?php
+
+/**
+ * Test the `wp_insert_link()` function.
+ *
+ * @group bookmark
+ * @covers ::wp_insert_link
+ */
+class Tests_Admin_Includes_Bookmark_wpInsertLink extends WP_UnitTestCase {
+
+	/**
+	 * Tests that wp_insert_link() inserts a link and assigns its category.
+	 *
+	 * @ticket 66019
+	 */
+	public function test_wp_insert_link_inserts_link_with_category() {
+		$category_id = self::factory()->term->create( array( 'taxonomy' => 'link_category' ) );
+
+		$link_id = wp_insert_link(
+			array(
+				'link_name'     => 'Inserted link',
+				'link_url'      => 'https://example.com',
+				'link_category' => array( $category_id ),
+			)
+		);
+
+		$this->assertIsInt( $link_id );
+		$this->assertSame( 'Inserted link', get_bookmark( $link_id )->link_name );
+		$this->assertSame( array( $category_id ), wp_get_link_cats( $link_id ) );
+	}
+
+	/**
+	 * Tests that wp_insert_link() rejects links without a URL.
+	 *
+	 * @ticket 66019
+	 * @dataProvider data_invalid_link_data
+	 *
+	 * @param array $link_data Invalid link data.
+	 */
+	public function test_wp_insert_link_rejects_invalid_data( $link_data ) {
+		$this->assertSame( 0, wp_insert_link( $link_data ) );
+	}
+
+	/**
+	 * Data provider for test_wp_insert_link_rejects_invalid_data().
+	 *
+	 * @return array<string, array{
+	 *     link_data: array<string, string>,
+	 * }>
+	 */
+	public function data_invalid_link_data(): array {
+		return array(
+			'no URL'         => array(
+				'link_data' => array( 'link_name' => 'Missing URL' ),
+			),
+			'no name or URL' => array(
+				'link_data' => array(),
+			),
+		);
+	}
+}
diff --git a/tests/phpunit/tests/admin/includes/bookmark/wpLinkManagerDisabledMessage.php b/tests/phpunit/tests/admin/includes/bookmark/wpLinkManagerDisabledMessage.php
new file mode 100644
index 0000000000000..5a903cf805e0f
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/bookmark/wpLinkManagerDisabledMessage.php
@@ -0,0 +1,23 @@
+<?php
+
+/**
+ * Test the `wp_link_manager_disabled_message()` function.
+ *
+ * @group bookmark
+ * @covers ::wp_link_manager_disabled_message
+ */
+class Tests_Admin_Includes_Bookmark_wpLinkManagerDisabledMessage extends WP_UnitTestCase {
+
+	/**
+	 * Tests that wp_link_manager_disabled_message() does nothing on other screens.
+	 *
+	 * @ticket 66019
+	 */
+	public function test_wp_link_manager_disabled_message_ignores_other_screens() {
+		global $pagenow;
+
+		$pagenow = 'index.php';
+
+		$this->assertNull( wp_link_manager_disabled_message() );
+	}
+}
diff --git a/tests/phpunit/tests/admin/includes/bookmark/wpSetLinkCats.php b/tests/phpunit/tests/admin/includes/bookmark/wpSetLinkCats.php
new file mode 100644
index 0000000000000..5d52065e61837
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/bookmark/wpSetLinkCats.php
@@ -0,0 +1,24 @@
+<?php
+
+/**
+ * Test the `wp_set_link_cats()` function.
+ *
+ * @group bookmark
+ * @covers ::wp_set_link_cats
+ */
+class Tests_Admin_Includes_Bookmark_wpSetLinkCats extends WP_UnitTestCase {
+
+	/**
+	 * Tests that wp_set_link_cats() assigns unique category IDs to a link.
+	 *
+	 * @ticket 66019
+	 */
+	public function test_wp_set_link_cats_assigns_unique_categories() {
+		$link_id      = self::factory()->bookmark->create();
+		$category_ids = self::factory()->term->create_many( 2, array( 'taxonomy' => 'link_category' ) );
+
+		wp_set_link_cats( $link_id, array( $category_ids[0], $category_ids[1], $category_ids[0] ) );
+
+		$this->assertSameSets( $category_ids, wp_get_link_cats( $link_id ) );
+	}
+}
diff --git a/tests/phpunit/tests/admin/includes/bookmark/wpUpdateLink.php b/tests/phpunit/tests/admin/includes/bookmark/wpUpdateLink.php
new file mode 100644
index 0000000000000..254751644c42e
--- /dev/null
+++ b/tests/phpunit/tests/admin/includes/bookmark/wpUpdateLink.php
@@ -0,0 +1,42 @@
+<?php
+
+/**
+ * Test the `wp_update_link()` function.
+ *
+ * @group bookmark
+ * @covers ::wp_update_link
+ */
+class Tests_Admin_Includes_Bookmark_wpUpdateLink extends WP_UnitTestCase {
+
+	/**
+	 * Tests that wp_update_link() updates link fields and categories.
+	 *
+	 * @ticket 66019
+	 */
+	public function test_wp_update_link_updates_link_data() {
+		$link_id     = self::factory()->bookmark->create(
+			array(
+				'link_name' => 'Original link',
+				'link_url'  => 'https://old.example.com',
+			)
+		);
+		$category_id = self::factory()->term->create( array( 'taxonomy' => 'link_category' ) );
+
+		$this->assertSame(
+			$link_id,
+			wp_update_link(
+				array(
+					'link_id'       => $link_id,
+					'link_name'     => 'Updated link',
+					'link_url'      => 'https://new.example.com',
+					'link_category' => array( $category_id ),
+				)
+			)
+		);
+
+		$link = get_bookmark( $link_id );
+		$this->assertSame( 'Updated link', $link->link_name );
+		$this->assertSame( 'https://new.example.com', $link->link_url );
+		$this->assertSame( array( $category_id ), wp_get_link_cats( $link_id ) );
+	}
+}