Allow ignoring PHPStan errors by PHP version

This is the approach PHPStan uses to ignore errors conditionally
based on the current PHP version

We will need this shortly to allow us to use code that uses enums,
while still performing analysis on PHP 7.1

Author: imjoehaines

.gitattributes

@@ -20,3 +20,4 @@
 /packager.php export-ignore
 /phpunit.xml.dist export-ignore
 /phpstan.neon.dist export-ignore
+/phpstan export-ignore

phpstan.neon.dist

@@ -1,3 +1,6 @@
+includes:
+    - phpstan/ignore-by-php-version.neon.php
+
 parameters:
     level: 6
 

phpstan/baseline-less-than-8.1.neon

@@ -0,0 +1,2 @@
+parameters:
+    ignoreErrors: []

phpstan/ignore-by-php-version.neon.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Ignore errors based on the current PHP version.
+ *
+ * This is useful in cases where errors are caused by the use of features that
+ * do not exist in the current version of PHP, e.g. enums
+ *
+ * This is the approach used by PHPStan itself:
+ * https://github.com/phpstan/phpstan-src/blob/master/build/ignore-by-php-version.neon.php
+ */
+
+use PHPStan\DependencyInjection\NeonAdapter;
+
+$config = [];
+$adapter = new NeonAdapter();
+
+if (version_compare(PHP_VERSION, '8.1.0', '<')) {
+    $config = array_merge_recursive($config, $adapter->load(__DIR__.'/baseline-less-than-8.1.neon'));
+}
+
+$config['parameters']['phpVersion'] = PHP_VERSION_ID;
+
+return $config;