Add static analysis checks to continuous integration

.github/workflows/ci.yml

@@ -44,3 +44,6 @@ jobs:
         env:
           CI_PHP_VERSION: ${{ matrix.php }}
           CI_PHP_FUTURE_RELEASE: ${{ matrix.future-release }}
+
+      - name: Static analysis
+        run: "vendor/bin/psalm"

composer.json

@@ -21,7 +21,8 @@
         "ext-gd": "*",
         "phpcompatibility/php-compatibility": "*",
         "phpunit/phpunit": "*",
-        "squizlabs/php_codesniffer": "*"
+        "squizlabs/php_codesniffer": "*",
+        "vimeo/psalm": "*"
     },
     "suggest": {
         "ext-mbstring": "*"

psalm.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<psalm
+    errorLevel="5"
+    resolveFromConfigFile="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>