fix: ensure that figure and figcaption elements have unique IDs

src/wp-includes/media.php

@@ -1,4 +1,4 @@
 <?php
 /**
  * WordPress API for media display.
  *
@@ -2599,8 +2599,8 @@
 	$describedby = '';
 
 	if ( $atts['id'] ) {
-		$atts['id'] = sanitize_html_class( $atts['id'] );
-		$id         = 'id="' . esc_attr( $atts['id'] ) . '" ';
+		$unique_id_value = preg_replace( '/-1$/', '', wp_unique_prefixed_id( sanitize_html_class( $atts['id'] . '-' ) ) );
+		$id              = 'id="' . esc_attr( $unique_id_value ) . '" ';
 	}
 
 	if ( $atts['caption_id'] ) {
@@ -2610,8 +2610,9 @@
 	}
 
 	if ( $atts['caption_id'] ) {
-		$caption_id  = 'id="' . esc_attr( $atts['caption_id'] ) . '" ';
-		$describedby = 'aria-describedby="' . esc_attr( $atts['caption_id'] ) . '" ';
+		$caption_id_value = preg_replace( '/-1$/', '', wp_unique_id( $atts['caption_id'] . '-' ) );
+		$caption_id       = 'id="' . esc_attr( $caption_id_value ) . '" ';
+		$describedby      = 'aria-describedby="' . esc_attr( $caption_id_value ) . '" ';
 	}
 
 	$class = trim( 'wp-caption ' . $atts['align'] . ' ' . $atts['class'] );

tests/phpunit/tests/media.php

@@ -206,7 +206,7 @@ public function test_img_caption_shortcode_with_old_format_id_and_align() {
 			)
 		);
 		$this->assertSame( 1, substr_count( $result, 'wp-caption &myAlignment' ) );
-		$this->assertSame( 1, substr_count( $result, 'id="myId"' ) );
+		$this->assertSame( 1, preg_match( '/id="myId[0-9]+"/', $result ) );
 		$this->assertSame( 1, substr_count( $result, self::CAPTION ) );
 	}
 
@@ -302,7 +302,95 @@ public function test_img_caption_shortcode_has_aria_describedby() {
 			self::IMG_CONTENT . self::HTML_CONTENT
 		);
 
-		$this->assertSame( 1, substr_count( $result, 'aria-describedby="caption-myId"' ) );
+		$this->assertSame( 1, preg_match( '/aria-describedby="caption-myId[0-9]+"/', $result ) );
+	}
+
+	/**
+	 * Tests that both figure and figcaption IDs are unique for multiple caption instances.
+	 *
+	 * When the same image with the same or different captions appears multiple
+	 * times on a page, each figure and figcaption should receive a unique ID to
+	 * maintain HTML validity and accessibility.
+	 *
+	 * @ticket 65315
+	 */
+	public function test_img_caption_shortcode_unique_ids_per_instance(): void {
+		// First instance with caption "My caption"
+		$result_1 = img_caption_shortcode(
+			array(
+				'width'   => 20,
+				'id'      => 'attachment_123',
+				'caption' => 'My caption',
+			),
+			self::IMG_CONTENT . 'My caption'
+		);
+
+		// Second instance - identical to first
+		$result_2 = img_caption_shortcode(
+			array(
+				'width'   => 20,
+				'id'      => 'attachment_123',
+				'caption' => 'My caption',
+			),
+			self::IMG_CONTENT . 'My caption'
+		);
+
+		// Third instance - same image, different caption
+		$result_3 = img_caption_shortcode(
+			array(
+				'width'   => 20,
+				'id'      => 'attachment_123',
+				'caption' => 'Different caption',
+			),
+			self::IMG_CONTENT . 'Different caption'
+		);
+
+		// Extract the figure (caption wrapper) and caption text IDs from each instance.
+		$figure_id_1  = $this->get_id_of_first_tag_with_class( $result_1, 'wp-caption' );
+		$figure_id_2  = $this->get_id_of_first_tag_with_class( $result_2, 'wp-caption' );
+		$figure_id_3  = $this->get_id_of_first_tag_with_class( $result_3, 'wp-caption' );
+		$caption_id_1 = $this->get_id_of_first_tag_with_class( $result_1, 'wp-caption-text' );
+		$caption_id_2 = $this->get_id_of_first_tag_with_class( $result_2, 'wp-caption-text' );
+		$caption_id_3 = $this->get_id_of_first_tag_with_class( $result_3, 'wp-caption-text' );
+
+		// Figure IDs should all exist
+		$this->assertNotEmpty( $figure_id_1, 'First figure should have an ID' );
+		$this->assertNotEmpty( $figure_id_2, 'Second figure should have an ID' );
+		$this->assertNotEmpty( $figure_id_3, 'Third figure should have an ID' );
+
+		// Figure IDs should all be different (each instance gets unique ID)
+		$this->assertNotSame( $figure_id_1, $figure_id_2, 'First and second figures should have different IDs even with identical content' );
+		$this->assertNotSame( $figure_id_2, $figure_id_3, 'Second and third figures should have different IDs' );
+		$this->assertNotSame( $figure_id_1, $figure_id_3, 'First and third figures should have different IDs' );
+
+		// Caption IDs should all exist
+		$this->assertNotEmpty( $caption_id_1, 'First caption should have an ID' );
+		$this->assertNotEmpty( $caption_id_2, 'Second caption should have an ID' );
+		$this->assertNotEmpty( $caption_id_3, 'Third caption should have an ID' );
+
+		// Caption IDs should all be different (each instance gets unique ID)
+		$this->assertNotSame( $caption_id_1, $caption_id_2, 'First and second captions should have different IDs even with identical content' );
+		$this->assertNotSame( $caption_id_2, $caption_id_3, 'Second and third captions should have different IDs' );
+		$this->assertNotSame( $caption_id_1, $caption_id_3, 'First and third captions should have different IDs' );
+	}
+
+	/**
+	 * Returns the `id` attribute of the first tag bearing the given class name.
+	 *
+	 * @param string $html       Markup to search.
+	 * @param string $class_name Class name to locate the tag by.
+	 * @return string|null The tag's `id` value, or null if no matching tag or `id` is found.
+	 */
+	private function get_id_of_first_tag_with_class( string $html, string $class_name ): ?string {
+		$processor = new WP_HTML_Tag_Processor( $html );
+
+		if ( ! $processor->next_tag( array( 'class_name' => $class_name ) ) ) {
+			return null;
+		}
+
+		$id = $processor->get_attribute( 'id' );
+
+		return is_string( $id ) ? $id : null;
 	}
 
 	public function test_add_remove_oembed_provider() {