fix for #643

Author: mevdschee

build.php

@@ -102,4 +102,10 @@ function run(string $base, array $dirs, string $filename, array $ignore)
     'vendor/nyholm/psr7/src/Factory/HttplugFactory.php',
 ];
 
-run(__DIR__, ['vendor/psr', 'vendor/nyholm', 'src'], 'api.php', $ignore);
+$directories = ['vendor/psr', 'vendor/nyholm', 'src'];
+
+if (extension_loaded('psr')) {
+    unset($directories['vendor/psr']);
+}
+
+run(__DIR__, $directories, 'api.php', $ignore);