mbstring: Make encoding detection stricter

PHP 8.3 changed how source encoding detection works:
https://www.php.net/manual/en/migration83.other-changes.php#migration83.other-changes.functions.mbstring
Most locales only consider `ASCII` and `UTF-8` (see `mb_detect_order()`),
and when a byte sequence invalid in both tested encodings (such as 0x91 for ‘ in Windows-1252) is encountered,
one of them might now be chosen as the most fitting encoding.
(This is done using the heuristics introduced in PHP 8.1:
php/php-src@28b346b)

Compare the output of the following script across PHP versions:

    <?php
    $result = hex2bin("91");
    var_dump(mb_detect_encoding($result));
    var_dump(mb_detect_encoding($result, 'auto', true));
    var_dump(mb_convert_encoding($result, 'UTF-8', 'auto'));

Let’s run the `mb_detect_encoding()` ourselves with `$strict` argument set to `true`, to ensure consistent behaviour across all PHP versions.
This might potentially cause a regression is some cases. Not sure.

Additionally, since we are now ensuring all encodings are valid, we can drop the warning capture mechanism.
It does not work on PHP ≥ 8.0 anyway, since that raises a `ValueError` instead of a warning when an invalid encoding is provided.
https://www.php.net/manual/en/function.mb-convert-encoding.php#refsect1-function.mb-convert-encoding-errors

Also adjust the confusing string in tests.

https://www.php.net/manual/en/function.mb-convert-encoding.php
https://www.php.net/manual/en/function.mb-detect-encoding.php
https://www.php.net/manual/en/function.mb-detect-order.php

Author: jtojnar

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [3.0.0] – unreleased
+
+- Mb: Make encoding detection stricter.
+
+
 ## [2.0.0] – 2023-03-07
 
 - Iconv: Fix warning on PHP 8.2 when passing `null` as source encoding.
@@ -23,5 +28,6 @@ The project has been revived and is now available under the name [`fossar/transc
 - Added Nix expression for easier development and sharing the environment with CI.
 - Switched to GitHub Actions for CI and added more PHP versions.
 
-[2.0.0]: https://github.com/fossar/transcoder/compare/1.0.1...v2.0.0
+[3.0.0]: https://github.com/fossar/transcoder/compare/v2.0.0...v3.0.0
+[2.0.0]: https://github.com/fossar/transcoder/compare/v1.0.1...v2.0.0
 [1.0.1]: https://github.com/fossar/transcoder/compare/1.0.0...v1.0.1

src/MbTranscoder.php

@@ -55,34 +55,31 @@ public function transcode(string $string, $from = null, ?string $to = null): str
             } else {
                 $this->assertSupported($from);
             }
+        } else {
+            $from = 'auto';
         }
 
         if ($to) {
             $this->assertSupported($to, false);
         }
 
-        $handleErrors = !$from || 'auto' === $from;
-        if ($handleErrors) {
-            set_error_handler(
-                function ($no, $warning) use ($string): void {
-                    throw new UndetectableEncodingException($string, $warning);
-                },
-                E_WARNING
-            );
+        if ($from === 'auto') {
+            $from = mb_detect_encoding($string, 'auto', true);
         }
 
-        try {
-            $result = mb_convert_encoding(
-                $string,
-                $to ?: $this->defaultEncoding,
-                $from ?: 'auto'
-            );
-        } finally {
-            if ($handleErrors) {
-                restore_error_handler();
-            }
+        if ($from === false) {
+            throw new UndetectableEncodingException($string, 'Unable to detect character encoding');
         }
 
+        $result = mb_convert_encoding(
+            $string,
+            $to ?: $this->defaultEncoding,
+            $from
+        );
+
+        // For PHPStan: We check the encoding is valid.
+        assert($result !== false);
+
         return $result;
     }
 

tests/MbTranscoderTest.php

@@ -55,7 +55,7 @@ public function testUndetectableEncoding(): void
         $this->expectException(\Ddeboer\Transcoder\Exception\UndetectableEncodingException::class);
         $this->expectExceptionMessage('is undetectable');
         $result = $this->transcoder->transcode(
-            '‘curly quotes make this incompatible with 1252’',
+            '‘Windows-1252 encodes curly quotes as 0x91 and 0x92, which are indistinguishable from any other single-byte encoding’',
             null,
             'windows-1252'
         );