Fix possible PHP warning in rss_enclosure()

src/wp-includes/feed.php

@@ -482,6 +482,10 @@ function rss_enclosure() {
 			foreach ( (array) $val as $enc ) {
 				$enclosure = explode( "\n", $enc );
 
+				if ( 3 > count( $enclosure ) ) {
+					continue;
+				}
+
 				// Only get the first element, e.g. 'audio/mpeg' from 'audio/mpeg mpga mp2 mp3'.
 				$t    = preg_split( '/[ \t]/', trim( $enclosure[2] ) );
 				$type = $t[0];

tests/phpunit/tests/feed/feed.php

@@ -0,0 +1,82 @@
+<?php
+
+/**
+ * Test the feed.
+ *
+ * @group feed
+ */
+class Tests_Feed extends WP_UnitTestCase {
+	protected static $enclosure_data = array(
+		'url'    => 'http://example.com/sound2.mp3',
+		'length' => 12345,
+		'type'   => 'audio/mpeg',
+	);
+
+	/**
+	 * Get the rss_enclosure() output.
+	 *
+	 * @return string
+	 */
+	protected function get_rss_enclosure() {
+		ob_start();
+		rss_enclosure();
+		return ob_get_clean();
+	}
+
+	/**
+	 * Get a multiline enclosure string.
+	 *
+	 * This function generates a multiline string like
+	 * wp_xmlrpc_server::add_enclosure_if_new function.
+	 *
+	 * @return string
+	 */
+	protected function get_multiline_enclosure_string() {
+		return self::$enclosure_data['url'] . "\n" . self::$enclosure_data['length'] . "\n" . self::$enclosure_data['type'] . "\n";
+	}
+
+	/**
+	 * @ticket 58798
+	 *
+	 * @covers rss_enclosure
+	 */
+	public function test_rss_enclosure_filter() {
+		$post_id         = self::factory()->post->create();
+		$GLOBALS['post'] = $post_id;
+
+		update_post_meta( $post_id, 'enclosure', $this->get_multiline_enclosure_string() );
+
+		add_filter(
+			'rss_enclosure',
+			function () {
+				return 'filtered_html_link_tag';
+			}
+		);
+
+		$this->assertSame( 'filtered_html_link_tag', $this->get_rss_enclosure(), 'The `rss_enclosure` filter could not be applied.' );
+	}
+
+	/**
+	 * @ticket 58798
+	 *
+	 * @covers rss_enclosure
+	 */
+	public function test_rss_enclosure() {
+		$this->assertEmpty( $this->get_rss_enclosure(), 'It should return empty when the global post is not set.' );
+
+		$post_id         = self::factory()->post->create();
+		$GLOBALS['post'] = $post_id;
+
+		$this->assertEmpty( $this->get_rss_enclosure(), 'The global post does not have the `enclosure` meta field and should return empty. ' );
+
+		update_post_meta( $post_id, 'enclosure', $this->get_multiline_enclosure_string() );
+
+		$expected = '<enclosure url="' . self::$enclosure_data['url'] . '" length="' . self::$enclosure_data['length'] . '" type="' . self::$enclosure_data['type'] . '" />' . "\n";
+
+		$this->assertSame( $expected, $this->get_rss_enclosure(), 'It should return a valid enclosure tag. ' );
+
+		update_post_meta( $post_id, 'enclosure', self::$enclosure_data['url'] );
+
+		$this->assertEmpty( $this->get_rss_enclosure(), 'It should return empty when the `enclosure` meta field is not saved in a multiline string.' );
+	}
+}