Handle invalid UTF-8 and a "DEL" ASCII code

In user output, e.g. for debugging, and in test failure messages there
may be invalid UTF-8 or an ASCII DEL character.

Invalid UTF-8 stopped PHP from producing JSON completely. This was found
while working on Rotational Cipher, where ASCII characters are rotated
and might result in invalid UTF-8.

ASCII DEL character was also produced during that exercise and resulted
in wrongly displayed output vs. JSON. PHPUnit detects ASCII control
chars and turns output to "binary string" - but not for invisible DEL
character (0x7F), which consumes the following character on screen.

src/Result.php

@@ -50,11 +50,25 @@ public function jsonSerialize(): mixed
         }
 
         if ($this->userOutput !== '') {
-            $result['output'] = $this->userOutput;
+            // In 2024 some innocent ASCII code still fools displays and editors.
+            $result['output'] = \str_replace(
+                [
+                    "\u{7F}", // Delete
+                ],
+                "\u{FFFD}", // Unicode substitute for invalid characters
+                $this->userOutput
+            );
         }
 
         if ($this->phpUnitMessage !== '') {
-            $result['message'] = $this->phpUnitMessage;
+            // In 2024 some innocent ASCII code still fools displays and editors.
+            $result['message'] = \str_replace(
+                [
+                    "\u{7F}", // Delete
+                ],
+                "\u{FFFD}", // Unicode substitute for invalid characters
+                $this->phpUnitMessage
+            );
         }
 
         return $result;

src/Tracer.php

@@ -165,7 +165,9 @@ private function saveResults(): void
             $this->outFileName,
             \json_encode(
                 $this->result,
-                self::DEBUG_PRETTY_JSON ? JSON_PRETTY_PRINT : 0
+                JSON_INVALID_UTF8_SUBSTITUTE | (
+                    self::DEBUG_PRETTY_JSON ? JSON_PRETTY_PRINT : 0
+                )
             ) . "\n",
         );
     }

tests/fail-with-invalid-chars-in-message/HelloWorld.php

@@ -0,0 +1,11 @@
+<?php
+
+function helloWorld()
+{
+    $invalidChars = [
+        "\u{7F}", // Delete
+        "\xFF\xFF\xFF\xFF\xFF\xFF", // Invalid UTF-8
+    ];
+
+    return 'Handle invalid chars: ' . implode(' ', $invalidChars) . '!';
+}

tests/fail-with-invalid-chars-in-message/HelloWorldTest.php

@@ -0,0 +1,16 @@
+<?php
+
+declare(strict_types=1);
+
+class HelloWorldTest extends PHPUnit\Framework\TestCase
+{
+    public static function setUpBeforeClass(): void
+    {
+        require_once 'HelloWorld.php';
+    }
+
+    public function testHelloWorld(): void
+    {
+        $this->assertEquals('Never matches', helloWorld());
+    }
+}

tests/fail-with-invalid-chars-in-message/expected_results.json

@@ -0,0 +1 @@
+{"version":3,"status":"fail","tests":[{"name":"Hello world","status":"fail","test_code":"$this->assertEquals('Never matches', helloWorld());\n","message":"HelloWorldTest::testHelloWorld\nFailed asserting that two strings are equal.\n--- Expected\n+++ Actual\n@@ @@\n-'Never matches'\n+'Handle invalid chars: \ufffd \ufffd\ufffd\ufffd\ufffd\ufffd\ufffd!'\n\nHelloWorldTest.php:14"}]}

tests/success-with-user-output/HelloWorld.php

@@ -2,7 +2,7 @@
 
 function helloWorld()
 {
-    echo "Some 'user üâ`|| \r\toutput\n"
+    echo "Some 'user üâ`|| \u{7F} \r\toutput \xFF\xFF\xFF\xFF\xFF\xFF \n"
         . 'containing \\ various "problematic" and UTF-8 chars' . PHP_EOL;
     var_dump(new stdClass());
 

tests/success-with-user-output/expected_results.json

@@ -1 +1 @@
-{"version":3,"status":"pass","tests":[{"name":"Hello world","status":"pass","test_code":"$this->assertEquals('Hello, World!', helloWorld());\n","output":"Some 'user \u00fc\u00e2`|| \r\toutput\ncontaining \\ various \"problematic\" and UTF-8 chars\nobject(stdClass)#79 (0) {\n}\n"}]}
+{"version":3,"status":"pass","tests":[{"name":"Hello world","status":"pass","test_code":"$this->assertEquals('Hello, World!', helloWorld());\n","output":"Some 'user \u00fc\u00e2`|| \ufffd \r\toutput \ufffd\ufffd\ufffd\ufffd\ufffd\ufffd \ncontaining \\ various \"problematic\" and UTF-8 chars\nobject(stdClass)#79 (0) {\n}\n"}]}