General: Replace core's remaining utf8_encode() calls.

src/wp-admin/includes/export.php

@@ -246,7 +246,7 @@ function wxr_cdata( $str ) {
 		$str = (string) $str;
 
 		if ( ! wp_is_valid_utf8( $str ) ) {
-			$str = utf8_encode( $str );
+			$str = _wp_iso_8859_1_to_utf8( $str );
 		}
 		// $str = ent2ncr(esc_html($str));
 		$str = '<![CDATA[' . str_replace( ']]>', ']]]]><![CDATA[>', $str ) . ']]>';

src/wp-admin/includes/image.php

@@ -1049,13 +1049,13 @@ function wp_read_image_metadata( $file ) {
 
 	foreach ( array( 'title', 'caption', 'credit', 'copyright', 'camera', 'iso' ) as $key ) {
 		if ( $meta[ $key ] && ! wp_is_valid_utf8( $meta[ $key ] ) ) {
-			$meta[ $key ] = utf8_encode( $meta[ $key ] );
+			$meta[ $key ] = _wp_iso_8859_1_to_utf8( $meta[ $key ] );
 		}
 	}
 
 	foreach ( $meta['keywords'] as $key => $keyword ) {
 		if ( ! wp_is_valid_utf8( $keyword ) ) {
-			$meta['keywords'][ $key ] = utf8_encode( $keyword );
+			$meta['keywords'][ $key ] = _wp_iso_8859_1_to_utf8( $keyword );
 		}
 	}
 

src/wp-includes/utf8.php

@@ -134,6 +134,48 @@ function wp_scrub_utf8( $text ) {
 	}
 endif;
 
+if ( extension_loaded( 'mbstring' ) ) :
+	/**
+	 * Converts a string from ISO-8859-1 (latin1) to UTF-8.
+	 *
+	 * This is a last resort for text whose encoding is unknown and which has already
+	 * failed to validate as UTF-8. Interpreting those bytes as ISO-8859-1 is a guess:
+	 * it is correct when the text really is ISO-8859-1, and produces mojibake for any
+	 * other single-byte or multi-byte encoding. Call sites which do know the encoding
+	 * of their text should decode it with {@see \mb_convert_encoding()} instead.
+	 *
+	 * Every byte maps to a code point, so the return value is always valid UTF-8.
+	 *
+	 * This function exists so that the call sites which historically relied on PHP’s
+	 * `utf8_encode()` can keep their behavior after that function’s removal in PHP 9.0.
+	 * It is not a replacement for {@see \wp_scrub_utf8()}, which neutralizes invalid
+	 * bytes instead of reinterpreting them.
+	 *
+	 * @ignore
+	 * @private
+	 *
+	 * @since 7.1.0
+	 *
+	 * @param string $text Text treated as ISO-8859-1 (latin1) bytes.
+	 * @return string Text converted into UTF-8.
+	 */
+	function _wp_iso_8859_1_to_utf8( $text ) {
+		return mb_convert_encoding( (string) $text, 'UTF-8', 'ISO-8859-1' );
+	}
+else :
+	/**
+	 * Fallback function for converting ISO-8859-1 into UTF-8.
+	 *
+	 * @ignore
+	 * @private
+	 *
+	 * @since 7.1.0
+	 */
+	function _wp_iso_8859_1_to_utf8( $text ) {
+		return _wp_utf8_encode_fallback( $text );
+	}
+endif;
+
 /**
  * Returns whether the given string contains Unicode noncharacters.
  *

tests/phpunit/tests/admin/exportWp.php

@@ -475,6 +475,94 @@ public function test_export_with_null_term_meta_values() {
 		$this->assertGreaterThan( 0, count( $xml->channel->item ), 'Export should contain items' );
 	}
 
+	/**
+	 * Ensures the WXR export converts non-UTF-8 text into UTF-8.
+	 *
+	 * The export declares itself as UTF-8, so `wxr_cdata()` has to hand back a valid
+	 * UTF-8 string. Text which fails to validate is decoded as ISO-8859-1 (latin1),
+	 * which is a guess: correct when the text really is latin1, mojibake otherwise.
+	 * Core has no encoding declaration to consult here, so the guess stands.
+	 *
+	 * These cases pin that behavior so it cannot change silently.
+	 *
+	 * @ticket 65828
+	 *
+	 * @dataProvider data_non_utf8_strings
+	 *
+	 * @param string $input    Bytes which are not valid UTF-8.
+	 * @param string $expected Expected CDATA contents.
+	 */
+	public function test_wxr_cdata_converts_non_utf8_text( $input, $expected ) {
+		// Running an export defines the nested WXR helper functions.
+		$this->get_the_export( array( 'content' => 'post' ) );
+
+		$actual = wxr_cdata( $input );
+		$inner  = substr( $actual, strlen( '<![CDATA[' ), -strlen( ']]>' ) );
+
+		$this->assertSame(
+			$expected,
+			$inner,
+			'Non-UTF-8 bytes should be decoded as ISO-8859-1.'
+		);
+		$this->assertTrue(
+			wp_is_valid_utf8( $inner ),
+			'The exported CDATA section should always contain valid UTF-8.'
+		);
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_non_utf8_strings() {
+		return array(
+			// Text which is well-formed, but in some encoding other than UTF-8.
+			'ISO-8859-1 text'                 => array(
+				mb_convert_encoding( 'Café', 'ISO-8859-1', 'UTF-8' ),
+				'Café',
+			),
+			'ISO-8859-2 text'                 => array(
+				mb_convert_encoding( 'wyróżnij', 'ISO-8859-2', 'UTF-8' ),
+				'wyró¿nij',
+			),
+			'Windows-1251 text'               => array(
+				mb_convert_encoding( 'Привет', 'Windows-1251', 'UTF-8' ),
+				'Ïðèâåò',
+			),
+			'Windows-1252 quotations'         => array(
+				mb_convert_encoding( '“quoted”', 'Windows-1252', 'UTF-8' ),
+				"\u{0093}quoted\u{0094}",
+			),
+
+			// Malformed UTF-8.
+			'Never-valid byte'                => array( "a\xC0b", 'aÀb' ),
+			'Truncated sequence'              => array( "a\xE2\x9Cb", "aâ\u{009C}b" ),
+			'Overlong sequence'               => array( "a\xC1\xBFb", 'aÁ¿b' ),
+			'Surrogate half'                  => array( "a\xED\xA0\x80b", "a\u{00ED}\u{00A0}\u{0080}b" ),
+
+			/*
+			 * The guard inspects the whole string, so a single stray byte sends the
+			 * valid portions through the conversion as well and double-encodes them.
+			 */
+			'Valid UTF-8 with one stray byte' => array( "Pi\xC3\xB1a \xC0", 'Piña À' ),
+		);
+	}
+
+	/**
+	 * Ensures valid UTF-8, including multibyte text, survives the export untouched.
+	 *
+	 * @ticket 65828
+	 */
+	public function test_wxr_cdata_preserves_valid_utf8() {
+		$this->get_the_export( array( 'content' => 'post' ) );
+
+		$valid = 'Это комментарий. / Βλέπετε ένα σχόλιο. / 🅰';
+		$inner = substr( wxr_cdata( $valid ), strlen( '<![CDATA[' ), -strlen( ']]>' ) );
+
+		$this->assertSame( $valid, $inner, 'Valid UTF-8 should pass through unchanged.' );
+	}
+
 	/**
 	 * Ensure that posts types with 'can_export' set to false are not included in the export.
 	 *

tests/phpunit/tests/image/meta.php

@@ -149,6 +149,96 @@ public function test_utf8_iptc_tags() {
 		$this->assertSame( 'This is a comment. / Это комментарий. / Βλέπετε ένα σχόλιο.', $out['caption'] );
 	}
 
+	/**
+	 * Ensures image metadata is always returned as valid UTF-8.
+	 *
+	 * Core does not read the IPTC coded character set, so the encoding of these fields
+	 * is unknown. Fields which fail to validate as UTF-8 are decoded as ISO-8859-1
+	 * (latin1): correct when the text really is latin1, mojibake otherwise. Invalid
+	 * UTF-8 reaching the database is stripped by `strip_invalid_text()`, which is why
+	 * the conversion has to happen at all. See #35316.
+	 *
+	 * These cases pin that behavior so it cannot change silently.
+	 *
+	 * @ticket 65828
+	 * @ticket 35316
+	 */
+	public function test_iptc_non_utf8_text_is_converted() {
+		if ( ! is_callable( 'iptcembed' ) || ! is_callable( 'iptcparse' ) ) {
+			$this->markTestSkipped( 'The iptcembed() and iptcparse() functions are required.' );
+		}
+
+		$block = $this->build_iptc_block(
+			array(
+				105 => mb_convert_encoding( 'wyróżnij', 'ISO-8859-2', 'UTF-8' ),
+				120 => mb_convert_encoding( 'Café', 'ISO-8859-1', 'UTF-8' ),
+				110 => "Credit \xC0",
+				116 => "Copyright \xE9",
+				25  => array(
+					'valid keyword',
+					"sunset\xC0",
+					// The Slovak keywords from the image attached to #35316.
+					mb_convert_encoding( 'Vodná elektráreň Gabčíkovo', 'ISO-8859-2', 'UTF-8' ),
+				),
+			)
+		);
+
+		$file = wp_tempnam( 'iptc-non-utf8.jpg' );
+		file_put_contents( $file, iptcembed( $block, DIR_TESTDATA . '/images/test-image.jpg' ) );
+
+		$out = wp_read_image_metadata( $file );
+
+		unlink( $file );
+
+		$this->assertIsArray( $out, 'Metadata should have been read from the image.' );
+
+		foreach ( array( 'title', 'caption', 'credit', 'copyright' ) as $key ) {
+			$this->assertTrue(
+				wp_is_valid_utf8( $out[ $key ] ),
+				"The '{$key}' field should always be valid UTF-8."
+			);
+		}
+
+		$this->assertSame(
+			'Café',
+			$out['caption'],
+			'ISO-8859-1 text should round-trip unchanged.'
+		);
+
+		$this->assertSame(
+			'wyró¿nij',
+			$out['title'],
+			'Text in another single-byte encoding should be decoded as ISO-8859-1.'
+		);
+
+		$this->assertSame(
+			array( 'valid keyword', 'sunsetÀ', 'Vodná elektráreò Gabèíkovo' ),
+			$out['keywords'],
+			'Keywords should be converted individually while valid entries are left alone.'
+		);
+	}
+
+	/**
+	 * Builds a raw IPTC APP13 block for the given record 2 datasets.
+	 *
+	 * @param array $tags Map of dataset number to a string value or list of string values.
+	 * @return string Binary IPTC block suitable for iptcembed().
+	 */
+	private function build_iptc_block( array $tags ) {
+		$block = '';
+
+		foreach ( $tags as $dataset => $values ) {
+			foreach ( (array) $values as $value ) {
+				$length = strlen( $value );
+				$block .= chr( 0x1C ) . chr( 2 ) . chr( $dataset )
+					. chr( ( $length >> 8 ) & 0xFF ) . chr( $length & 0xFF )
+					. $value;
+			}
+		}
+
+		return $block;
+	}
+
 	/**
 	 * wp_read_image_metadata() should return false if the image file doesn't exist.
 	 */