Allow installing under PHP 8

1. Allow PHPUnit 9 (needed for PHP 8)

2. Shim deprecated libxml functions

Make the call to libxml_disable_entity_loader conditional.  This
function has been deprecated in PHP 8.0 and the new default is
equivalent to calling that function with `true`.

https://php.watch/versions/8.0/libxml_disable_entity_loader-deprecation

Note: there were two instances of exactly the same XML-parsing anonymous
function. I've traced this duplication back to 2015 when the functions
were one line long and explicitness might have been better than
deduplication. Over the years these functions have been growing together
and I see no obvious reason why two copies should be maintained.

composer.json

@@ -28,7 +28,7 @@
     }
   ],
   "require": {
-    "php": "^7.2",
+    "php": "^7.2 || ^8.0",
     "ext-SimpleXML": "*",
     "ext-fileinfo": "*",
     "ext-json": "*",
@@ -42,7 +42,7 @@
     "nyholm/psr7": "^1.3",
     "php-http/psr7-integration-tests": "dev-master",
     "phpstan/phpstan": "^0.12.52",
-    "phpunit/phpunit": "^8.5",
+    "phpunit/phpunit": "^8.5 || ^9.3",
     "squizlabs/php_codesniffer": "^3.5"
   },
   "autoload": {

src/ServerRequest.php

@@ -61,12 +61,12 @@ final public function __construct(ServerRequestInterface $serverRequest)
             return $result;
         });
 
-        $this->registerMediaTypeParser('application/xml', function ($input) {
-            $backup = libxml_disable_entity_loader(true);
+        $xmlParserCallable = function ($input) {
+            $backup = self::disableXmlEntityLoader(true);
             $backup_errors = libxml_use_internal_errors(true);
             $result = simplexml_load_string($input);
 
-            libxml_disable_entity_loader($backup);
+            self::disableXmlEntityLoader($backup);
             libxml_clear_errors();
             libxml_use_internal_errors($backup_errors);
 
@@ -75,23 +75,10 @@ final public function __construct(ServerRequestInterface $serverRequest)
             }
 
             return $result;
-        });
-
-        $this->registerMediaTypeParser('text/xml', function ($input) {
-            $backup = libxml_disable_entity_loader(true);
-            $backup_errors = libxml_use_internal_errors(true);
-            $result = simplexml_load_string($input);
-
-            libxml_disable_entity_loader($backup);
-            libxml_clear_errors();
-            libxml_use_internal_errors($backup_errors);
-
-            if ($result === false) {
-                return null;
-            }
+        };
 
-            return $result;
-        });
+        $this->registerMediaTypeParser('application/xml', $xmlParserCallable);
+        $this->registerMediaTypeParser('text/xml', $xmlParserCallable);
 
         $this->registerMediaTypeParser('application/x-www-form-urlencoded', function ($input) {
             parse_str($input, $data);
@@ -781,4 +768,17 @@ public function isXhr(): bool
     {
         return $this->serverRequest->getHeaderLine('X-Requested-With') === 'XMLHttpRequest';
     }
+
+    private static function disableXmlEntityLoader(bool $disable): bool
+    {
+        if (\LIBXML_VERSION >= 20900) {
+            // libxml >= 2.9.0 disables entity loading by default, so it is
+            // safe to skip the real call (deprecated in PHP 8).
+            return true;
+        }
+
+        // @codeCoverageIgnoreStart
+        return libxml_disable_entity_loader($disable);
+        // @codeCoverageIgnoreEnd
+    }
 }