Don't manipulate keys, throw exceptions instead.

Replacing special characters can result in cache key collisions which
can lead to unexpected results. Instead we can throw exceptions on
invalid data.

src/Cache/Engine/FileEngine.php

@@ -16,7 +16,7 @@ declare(strict_types=1);
 namespace Cake\Cache\Engine;
 
 use Cake\Cache\CacheEngine;
-use Cake\Utility\Inflector;
+use Cake\Cache\InvalidArgumentException;
 use CallbackFilterIterator;
 use Exception;
 use LogicException;
@@ -422,11 +422,14 @@ class FileEngine extends CacheEngine
     {
         $key = parent::_key($key);
 
-        return Inflector::underscore(str_replace(
-            [DIRECTORY_SEPARATOR, '/', '.', '<', '>', '?', ':', '|', '*', '"'],
-            '_',
-            (string)$key
-        ));
+        if (preg_match('/[\/\\<>?:|*"]/', (string)$key)) {
+            throw new InvalidArgumentException(
+                "Cache key `{$key}` contains invalid characters. " .
+                'You cannot use /, \\, <, >, ?, :, |, *, or " in cache keys.'
+            );
+        }
+
+        return $key;
     }
 
     /**

tests/TestCase/Cache/Engine/FileEngineTest.php

@@ -16,6 +16,7 @@ declare(strict_types=1);
 namespace Cake\Test\TestCase\Cache\Engine;
 
 use Cake\Cache\Cache;
+use Cake\Cache\InvalidArgumentException;
 use Cake\Cache\Engine\FileEngine;
 use Cake\Core\Configure;
 use Cake\TestSuite\TestCase;
@@ -292,21 +293,37 @@ class FileEngineTest extends TestCase
     {
         $result = Cache::write('views.countries.something', 'here', 'file_test');
         $this->assertTrue($result);
-        $this->assertFileExists(TMP . 'tests/cake_views_countries_something');
+        $this->assertFileExists(TMP . 'tests/cake_views.countries.something');
 
         $result = Cache::read('views.countries.something', 'file_test');
         $this->assertEquals('here', $result);
 
         $result = Cache::clear('file_test');
         $this->assertTrue($result);
+    }
 
-        $result = Cache::write('domain.test.com:8080', 'here', 'file_test');
-        $this->assertTrue($result);
-        $this->assertFileExists(TMP . 'tests/cake_domain_test_com_8080');
+    /**
+     * Test invalid key() containing :
+     *
+     * @return void
+     */
+    public function testInvalidKeyColon()
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('contains invalid characters');
+        Cache::write('domain.test.com:8080', 'here', 'file_test');
+    }
 
-        $result = Cache::write('command>dir|more', 'here', 'file_test');
-        $this->assertTrue($result);
-        $this->assertFileExists(TMP . 'tests/cake_command_dir_more');
+    /**
+     * Test invalid key() containing >
+     *
+     * @return void
+     */
+    public function testInvalidKeyAngleBracket()
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('contains invalid characters');
+        Cache::write('command>dir|more', 'here', 'file_test');
     }
 
     /**