Add Psalm for static analysis

.gitignore

@@ -5,5 +5,6 @@
 !/.gitignore
 !/composer.json
 !/Makefile
+!/psalm.xml
 !/src/**
 !/tests/**

Makefile

@@ -1,5 +1,8 @@
 .PHONY: *
 
+static-code-analysis:
+	php vendor/bin/psalm --show-info=true
+
 phpcs:
 	php vendor/bin/phpcs --standard=PSR2 src tests
 

composer.json

@@ -26,7 +26,8 @@
     "require-dev": {
         "phpunit/phpunit": "^9.1",
         "squizlabs/php_codesniffer": "^3.5",
-        "symfony/var-dumper": "^5.0"
+        "symfony/var-dumper": "^5.0",
+        "vimeo/psalm": "^3.11"
     },
     "config": {
         "sort-packages": true

psalm.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0"?>
+<psalm
+    errorLevel="7"
+    findUnusedVariablesAndParams="true"
+    resolveFromConfigFile="true"
+    totallyTyped="true"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns="https://getpsalm.org/schema/config"
+    xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
+>
+    <projectFiles>
+        <directory name="src" />
+        <ignoreFiles>
+            <directory name="vendor" />
+        </ignoreFiles>
+    </projectFiles>
+</psalm>