Avoid using curl when it has been disabled, fixes #9423

src/Composer/Command/DiagnoseCommand.php

@@ -379,7 +379,11 @@ EOT
 
     private function getCurlVersion()
     {
-        if (function_exists('curl_version')) {
+        if (extension_loaded('curl')) {
+            if (!HttpDownloader::isCurlEnabled()) {
+                return '<error>disabled via disable_functions, using php streams fallback, which reduces performance</error>';
+            }
+
             $version = curl_version();
 
             return '<comment>'.$version['version'].'</comment> '.

src/Composer/Util/HttpDownloader.php

@@ -67,8 +67,7 @@ class HttpDownloader
         $this->options = array_replace_recursive($this->options, $options);
         $this->config = $config;
 
-        // TODO enable curl only on 5.6+ if older versions cause any problem
-        if (extension_loaded('curl')) {
+        if (self::isCurlEnabled()) {
             $this->curl = new Http\CurlDownloader($io, $config, $options, $disableTls);
         }
 
@@ -423,4 +422,9 @@ class HttpDownloader
 
         return true;
     }
+
+    public static function isCurlEnabled()
+    {
+        return \extension_loaded('curl') && \function_exists('curl_multi_exec') && \function_exists('curl_multi_init');
+    }
 }