diff --git a/php/src/GildedRose.php b/php/src/GildedRose.php
index ef44ade3..65e6c058 100644
--- a/php/src/GildedRose.php
+++ b/php/src/GildedRose.php
@@ -35,6 +35,10 @@ final class GildedRose
             ・計算後sell_inが10未満の場合、さらにquality+1する
             ・計算後sell_inが5未満の場合、さらにquality+1する
             ・計算後sell_inが0未満の場合、qualityを0にする
+        [その他商品]
+        ・引数のsell_inを-1する
+        ・引数のqualityを-1する
+            ・sell_inが0未満の場合、さらにquality-1する ★仕様書に記載なかったがコード上はこのようになっている
     */
 
     /**
@@ -52,6 +56,9 @@ final class GildedRose
             } elseif ($this->item->name === 'Backstage passes to a TAFKAL80ETC concert') {
                 // 商品：Backstage passesの処理
                 $this->backstagePasses();
+            } else {
+                // その他商品の処理
+                $this->others();
             }
         }
     }
@@ -100,6 +107,20 @@ final class GildedRose
         }
     }
 
+    /**
+     * その他商品
+     */
+    private function others(): void
+    {
+        $this->calcSellInSubtraction();
+        $this->calcQualitySubtraction();
+
+        // sell_inが0未満の場合、sell_inを再減算する
+        if ($this->item->sell_in < 0) {
+            $this->calcQualitySubtraction();
+        }
+    }
+
     /**
      * sell_inの減算を行う
      */
@@ -118,5 +139,16 @@ final class GildedRose
             ++$this->item->quality;
         }
     }
+
+    /**
+     * qualityの減算を行う
+     */
+    private function calcQualitySubtraction(): void
+    {
+        // 1以上の場合計算
+        if ($this->item->quality >= 1) {
+            --$this->item->quality;
+        }
+    }
 }
 
diff --git a/php/tests/GildedRoseTest.php b/php/tests/GildedRoseTest.php
index 0b6408b5..82f1a3a4 100644
--- a/php/tests/GildedRoseTest.php
+++ b/php/tests/GildedRoseTest.php
@@ -193,6 +193,55 @@ class GildedRoseTest extends TestCase
         $this->assertSame(50, $items[0]->quality);
     }
 
+    /**
+     * Foo：sell_inが1以上、qualityが50未満
+     * 期待値：sell_inが-1、qualityが-1
+     */
+    public function testFooNormal(): void
+    {
+        $items = [new Item('Foo', 5, 10)];
+        $gildedRose = new GildedRose($items);
+        $gildedRose->updateQuality();
+        $this->assertSame('Foo', $items[0]->name);
+        $this->assertSame(4, $items[0]->sell_in);
+        $this->assertSame(9, $items[0]->quality);
+    }
+
+    /**
+     * Foo：sell_inが0以下、qualityが50未満
+     * 期待値：sell_inが-1、qualityが-2
+     */
+    public function testFooSellIn0OrLess(): void
+    {
+        $items = [new Item('Foo', 0, 10)];
+        $gildedRose = new GildedRose($items);
+        $gildedRose->updateQuality();
+        $this->assertSame('Foo', $items[0]->name);
+        $this->assertSame(-1, $items[0]->sell_in);
+        $this->assertSame(8, $items[0]->quality);
+
+        $items = [new Item('Foo', -1, 10)];
+        $gildedRose = new GildedRose($items);
+        $gildedRose->updateQuality();
+        $this->assertSame('Foo', $items[0]->name);
+        $this->assertSame(-2, $items[0]->sell_in);
+        $this->assertSame(8, $items[0]->quality);
+    }
+
+    /**
+     * Foo：sell_inが1以上、qualityが0
+     * 期待値：sell_inが-1、qualityは変更なし
+     */
+    public function testFooQuality0(): void
+    {
+        $items = [new Item('Foo', 5, 0)];
+        $gildedRose = new GildedRose($items);
+        $gildedRose->updateQuality();
+        $this->assertSame('Foo', $items[0]->name);
+        $this->assertSame(4, $items[0]->sell_in);
+        $this->assertSame(0, $items[0]->quality);
+    }
+
     /**
      * 複数商品
      */
@@ -202,6 +251,7 @@ class GildedRoseTest extends TestCase
             new Item('Aged Brie', 5, 10),
             new Item('Sulfuras, Hand of Ragnaros', 5, 80),
             new Item('Backstage passes to a TAFKAL80ETC concert', 5, 10),
+            new Item('Foo', 5, 10),
         ];
         $gildedRose = new GildedRose($items);
         $gildedRose->updateQuality();
@@ -214,6 +264,9 @@ class GildedRoseTest extends TestCase
         $this->assertSame('Backstage passes to a TAFKAL80ETC concert', $items[2]->name);
         $this->assertSame(4, $items[2]->sell_in);
         $this->assertSame(13, $items[2]->quality);
+        $this->assertSame('Foo', $items[3]->name);
+        $this->assertSame(4, $items[3]->sell_in);
+        $this->assertSame(9, $items[3]->quality);
     }
 
     // テストエラーの原因が特定できないので後で調査する