Use the native implementation for Memcached::decrement

hphp/runtime/ext/memcached/ext_memcached.php

@@ -204,12 +204,11 @@ public function casByKey(float $cas_token,
    *
    * @return int - Returns item's new value on success.
    */
+  <<__Native>>
   public function decrement(string $key,
                             int $offset = 1,
                             int $initial_value = 0,
-                            int $expiry = 0): mixed {
-    return $this->decrementByKey('', $key, $offset, $initial_value, $expiry);
-  }
+                            int $expiry = 0): mixed;
 
   /**
    * Decrement numeric item's value, stored on a specific server

hphp/test/slow/ext_memcached/dec_key.php

@@ -0,0 +1,11 @@
+<?php
+$mc = new Memcached;
+$mc->addServer('127.0.0.1', 11211);
+$mc->addServer('127.0.0.1', 11212);
+for ($i = 1; $i <= 10; $i++) {
+    $key = "dec_test_{$i}";
+    $mc->set($key, 10);
+    var_dump($mc->get($key) === 10);
+    $mc->decrement($key, 1);
+    var_dump($mc->get($key) === 9);
+}

hphp/test/slow/ext_memcached/dec_key.php.expect

@@ -0,0 +1,20 @@
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)

hphp/test/slow/ext_memcached/dec_key.php.skipif

@@ -0,0 +1,9 @@
+<?php
+
+$memc = new Memcached();
+$memc->addServer('localhost', '11211');
+$memc->addServer('localhost', '11212');
+$version = $memc->getVersion();
+if (!is_array($version) || count($version) !== 2) {
+  echo "SKIP Need two Memcached servers running (port 11211 and 11212)";
+}