fix: return CURLM_OK from intercepted curl_multi_add_handle

src/VCR/LibraryHooks/CurlHook.php

@@ -184,13 +184,18 @@ public static function curlExec(\CurlHandle $curlHandle)
      *
      * @see http://www.php.net/manual/en/function.curl-multi-add-handle.php
      */
-    public static function curlMultiAddHandle(\CurlMultiHandle $multiHandle, \CurlHandle $curlHandle): void
+    public static function curlMultiAddHandle(\CurlMultiHandle $multiHandle, \CurlHandle $curlHandle): int
     {
         if (!isset(self::$multiHandles[(int) $multiHandle])) {
             self::$multiHandles[(int) $multiHandle] = [];
         }
 
         self::$multiHandles[(int) $multiHandle][(int) $curlHandle] = $curlHandle;
+
+        // Mirror the real curl_multi_add_handle() return contract. Callers such as
+        // Guzzle >= 7.13 strictly check `\CURLM_OK !== $result` and throw otherwise;
+        // returning void (null) here made that check fail with "No error (0)".
+        return \CURLM_OK;
     }
 
     /**

tests/Unit/LibraryHooks/CurlHookTest.php

@@ -441,6 +441,31 @@ function (Request $request) use ($testClass, &$callCount) {
         );
     }
 
+    public function testCurlMultiAddHandleReturnsCurlmOk(): void
+    {
+        $this->curlHook->enable(
+            fn (Request $request): Response => new Response('200')
+        );
+
+        $curlHandle = curl_init('http://example.com');
+        Assertion::notSame($curlHandle, false);
+
+        $curlMultiHandle = curl_multi_init();
+        Assertion::notSame($curlMultiHandle, false);
+
+        // Real curl_multi_add_handle() returns CURLM_OK (0) on success. Guzzle >= 7.13
+        // strictly checks `\CURLM_OK !== $result` and throws when it is not returned,
+        // so the intercepted call must mirror that contract rather than return void.
+        $result = curl_multi_add_handle($curlMultiHandle, $curlHandle);
+
+        curl_multi_remove_handle($curlMultiHandle, $curlHandle);
+        curl_multi_close($curlMultiHandle);
+
+        $this->curlHook->disable();
+
+        $this->assertSame(\CURLM_OK, $result, 'Intercepted curl_multi_add_handle should return CURLM_OK.');
+    }
+
     /**
      * @requires PHP 5.5.0
      */