[8.x] Pass $key to closure in Collection and LazyCollection's reduce method as well

src/Illuminate/Collections/Collection.php

@@ -881,7 +881,13 @@ public function random($number = null)
      */
     public function reduce(callable $callback, $initial = null)
     {
-        return array_reduce($this->items, $callback, $initial);
+        $result = $initial;
+
+        foreach ($this->items as $key => $value) {
+            $result = $callback($result, $value, $key);
+        }
+
+        return $result;
     }
 
     /**

src/Illuminate/Collections/LazyCollection.php

@@ -838,8 +838,8 @@ public function reduce(callable $callback, $initial = null)
     {
         $result = $initial;
 
-        foreach ($this as $value) {
-            $result = $callback($result, $value);
+        foreach ($this as $key => $value) {
+            $result = $callback($result, $value, $key);
         }
 
         return $result;

tests/Support/SupportCollectionTest.php

@@ -3601,7 +3601,7 @@ public function testReduceWithKeys($collection)
             'foo' => 'bar',
             'baz' => 'qux',
         ]);
-        $this->assertEquals('foobarbazqux', $data->reduceWithKeys(function ($carry, $element, $key) {
+        $this->assertEquals('foobarbazqux', $data->reduce(function ($carry, $element, $key) {
             return $carry .= $key.$element;
         }));
     }