[HttpClient] make HttpClient::create() return an AmpHttpClient when amphp/http-client is found but curl is not or too old

Author: alexandre-daubois

http_client.rst

@@ -854,14 +854,26 @@ To leverage all these design benefits, the cURL extension is needed.
 Enabling cURL Support
 ~~~~~~~~~~~~~~~~~~~~~
 
-This component supports both the native PHP streams and cURL to make the HTTP
-requests. Although both are interchangeable and provide the same features,
-including concurrent requests, HTTP/2 is only supported when using cURL.
+This component supports the native PHP streams, `AmpHttpClient`_ and cURL to
+make the HTTP requests. Although they are interchangeable and provide the
+same features, including concurrent requests, HTTP/2 is only supported when
+using cURL or AmpHttpClient.
+
+.. note::
+
+    To use the `AmpHttpClient`_, the `amphp/http-client`_ package must be
+    installed.
+
+.. versionadded:: 5.1
+
+    Integration with ``amphp/http-client`` was introduced in Symfony 5.1.
 
 The :method:`Symfony\\Component\\HttpClient\\HttpClient::create` method
-selects the cURL transport if the `cURL PHP extension`_ is enabled and falls
-back to PHP streams otherwise. If you prefer to select the transport
-explicitly, use the following classes to create the client::
+selects the cURL transport if the `cURL PHP extension`_ is enabled. It falls
+back to ``AmpHttpClient`` if cURL couldn't be found or is too old. Finally, if
+``AmpHttpClient`` is not available, it falls back to PHP streams.
+If you prefer to select the transport explicitly, use the following classes
+to create the client::
 
     use Symfony\Component\HttpClient\CurlHttpClient;
     use Symfony\Component\HttpClient\NativeHttpClient;
@@ -874,7 +886,7 @@ explicitly, use the following classes to create the client::
 
 When using this component in a full-stack Symfony application, this behavior is
 not configurable and cURL will be used automatically if the cURL PHP extension
-is installed and enabled. Otherwise, the native PHP streams will be used.
+is installed and enabled, and will fall back as explained above.
 
 Configuring CurlHttpClient Options
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -2055,3 +2067,4 @@ test it in a real application::
 .. _`EventSource`: https://www.w3.org/TR/eventsource/#eventsource
 .. _`idempotent method`: https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol#Idempotent_methods
 .. _`SSRF`: https://portswigger.net/web-security/ssrf
+.. _`AmpHttpClient`: https://github.com/amphp/http-client