fix: MemcachedHandler::decrement() wrong sign on missing key

MemcachedHandler::decrement() passed $offset as Memcached::decrement()'s
initial_value, which is used as-is (not decremented from) when a key
doesn't exist yet. That made a fresh key end up at +$offset instead of
the -$offset every other cache handler (File, Redis, Predis) produces.

Memcached counters are unsigned, so they can't hold a negative initial
value the way the other handlers effectively can. Use 0 instead (also
Memcached::decrement()'s own default), so a fresh key at least stops
going the wrong direction.

Updates the existing MemcachedHandlerTest::testDecrement() expectation
and adds a changelog entry.

Author: mdalikadar

system/Cache/Handlers/MemcachedHandler.php

@@ -180,9 +180,11 @@ public function decrement(string $key, int $offset = 1): false|int
 
         $key = static::validateKey($key, $this->prefix);
 
-        // FIXME: third parameter isn't other handler actions.
-
-        return $this->memcached->decrement($key, $offset, $offset, 60);
+        // Memcached counters are unsigned, so a missing key can't be
+        // initialized to a negative value like the other handlers do.
+        // Fall back to Memcached::decrement()'s own default of 0 instead
+        // of $offset, so a new key no longer starts positive.
+        return $this->memcached->decrement($key, $offset, 0, 60);
     }
 
     public function clean(): bool

tests/system/Cache/Handlers/MemcachedHandlerTest.php

@@ -151,7 +151,9 @@ public function testDecrement(): void
 
         $this->assertSame(9, $memcachedHandler->decrement(self::$key1, 1));
         $this->assertFalse($memcachedHandler->decrement(self::$key2, 1));
-        $this->assertSame(1, $memcachedHandler->decrement(self::$key3, 1));
+        // A key that doesn't exist yet starts at 0, not at the offset
+        // (Memcached counters are unsigned, so it can't start negative).
+        $this->assertSame(0, $memcachedHandler->decrement(self::$key3, 1));
     }
 
     public function testClean(): void

user_guide_src/source/changelogs/v4.7.5.rst

@@ -39,6 +39,7 @@ Bugs Fixed
 - **Helpers:** Fixed a bug where ``get_dir_file_info()`` returned incomplete entries for subdirectories and missing files instead of omitting them.
 - **Honeypot:** Fixed a bug where bot detection returned an HTTP 500 response instead of 403 (Forbidden).
 - **Logger:** Fixed a bug where interpolating a log message with array or non-stringable context values could raise PHP warnings or errors.
+- **Cache:** Fixed a bug where ``MemcachedHandler::decrement()`` on a non-existent key returned a positive value instead of ``0``, the opposite sign of what ``FileHandler``, ``RedisHandler``, and ``PredisHandler`` return.
 
 See the repo's
 `CHANGELOG.md <https://github.com/codeigniter4/CodeIgniter4/blob/develop/CHANGELOG.md>`_