diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 6fe9d7b..b6d9a84 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -18,6 +18,3 @@ jobs:
     uses: matomo-org/plugin-ci-workflows/.github/workflows/plugin-ci.yml@main
     with:
       plugin-name: CustomVariables
-      # PHPStan has never run here and there is no phpstan.neon to run it with;
-      # adding one is a decision of its own.
-      skip-phpstan: true
diff --git a/API.php b/API.php
index c8066a4..08cb9c3 100644
--- a/API.php
+++ b/API.php
@@ -14,7 +14,6 @@
 use Piwik\Archive;
 use Piwik\Container\StaticContainer;
 use Piwik\DataTable;
-use Piwik\Date;
 use Piwik\Metrics;
 use Piwik\Piwik;
 
@@ -28,21 +27,21 @@ class API extends \Piwik\Plugin\API
     /**
      * @param int $idSite
      * @param string $period
-     * @param Date $date
+     * @param string $date
      * @param string $segment
      * @param bool $expanded
-     * @param int $idSubtable
+     * @param int|null $idSubtable
      *
      * @return DataTable|DataTable\Map
      */
     protected function getDataTable($idSite, $period, $date, $segment, $expanded, $flat, $idSubtable)
     {
         $dataTable = Archive::createDataTableFromArchive(Archiver::CUSTOM_VARIABLE_RECORD_NAME, $idSite, $period, $date, $segment, $expanded, $flat, $idSubtable);
-        $dataTable->queueFilter('ColumnDelete', 'nb_uniq_visitors');
+        $dataTable->queueFilter('ColumnDelete', ['nb_uniq_visitors']);
 
         if ($flat) {
             $dataTable->filterSubtables('Sort', array(Metrics::INDEX_NB_ACTIONS, 'desc', $naturalSort = false, $expanded));
-            $dataTable->queueFilterSubtables('ColumnDelete', 'nb_uniq_visitors');
+            $dataTable->queueFilterSubtables('ColumnDelete', ['nb_uniq_visitors']);
         }
 
         return $dataTable;
@@ -166,9 +165,9 @@ public function getUsagesOfSlots($idSite)
             'page'  => array_fill(1, $numVars, array()),
         );
 
-        /** @var DataTable $customVarUsages */
         $today = StaticContainer::get('CustomVariables.today');
         $date = '2008-12-12,' . $today;
+        /** @var DataTable $customVarUsages */
         $customVarUsages = Request::processRequest(
             'CustomVariables.getCustomVariables',
             array('idSite' => $idSite, 'period' => 'range', 'date' => $date,
diff --git a/RecordBuilders/CustomVariables.php b/RecordBuilders/CustomVariables.php
index 890cc09..46224ff 100644
--- a/RecordBuilders/CustomVariables.php
+++ b/RecordBuilders/CustomVariables.php
@@ -74,7 +74,7 @@ protected function aggregate(ArchiveProcessor $archiveProcessor): array
     }
 
 
-    protected function aggregateCustomVariable(DataTable $record, array &$metadata, array &$metadataFlat, LogAggregator $logAggregator, string $slot): void
+    protected function aggregateCustomVariable(DataTable $record, array &$metadata, array &$metadataFlat, LogAggregator $logAggregator, int $slot): void
     {
         $keyField = "custom_var_k" . $slot;
         $valueField = "custom_var_v" . $slot;
diff --git a/Reports/GetCustomVariables.php b/Reports/GetCustomVariables.php
index 0f01ce2..448c10a 100644
--- a/Reports/GetCustomVariables.php
+++ b/Reports/GetCustomVariables.php
@@ -71,9 +71,7 @@ public function getFooterMessageExplanationMissingMetrics()
             // no footer message for subtables
             $out = '';
             Piwik::postEvent('Template.afterCustomVariablesReport', array(&$out));
-            if (!empty($message)) {
-                $message .= $out;
-            }
+            $message .= $out;
         }
 
         return $message;
diff --git a/Tracker/CustomVariablesRequestProcessor.php b/Tracker/CustomVariablesRequestProcessor.php
index e7752eb..fe15ded 100644
--- a/Tracker/CustomVariablesRequestProcessor.php
+++ b/Tracker/CustomVariablesRequestProcessor.php
@@ -46,6 +46,8 @@ public function processRequestParams(VisitProperties $visitProperties, Request $
         }
 
         $request->setMetadata('CustomVariables', 'visitCustomVariables', $visitorCustomVariables);
+
+        return false;
     }
 
     public function onNewVisit(VisitProperties $visitProperties, Request $request)
@@ -71,7 +73,7 @@ public function afterRequestProcessed(VisitProperties $visitProperties, Request
         $action = $request->getMetadata('Actions', 'action');
 
         if (empty($action) || !($action instanceof Action)) {
-            return;
+            return false;
         }
 
         $customVariables = self::getCustomVariablesInPageScope($request);
@@ -84,6 +86,8 @@ public function afterRequestProcessed(VisitProperties $visitProperties, Request
                 $action->setCustomField($field, $value);
             }
         }
+
+        return false;
     }
 
     public static function getCustomVariablesInVisitScope(Request $request)
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 0000000..e8f7e2b
--- /dev/null
+++ b/phpstan.neon
@@ -0,0 +1,35 @@
+parameters:
+    level: 5
+    phpVersion: 80100
+    tmpDir: /tmp/phpstan/CustomVariables/main
+    paths:
+        - .
+    excludePaths:
+        analyseAndScan:
+            - tests/*
+            # The shared workflow leaves this helper checkout in the workspace, and `paths: .`
+            # would otherwise analyse it as if it were plugin code.
+            - github-action-tests/*
+    ignoreErrors:
+        # Row::setColumn() and deleteColumn() are untyped and only document `@param string`, while
+        # archiving addresses columns by the integer Metrics::INDEX_* constants throughout. Pinned
+        # to the five call sites that do so, so a sixth has to be looked at rather than absorbed.
+        -
+            identifier: argument.type
+            message: '#Row::(setColumn|deleteColumn)\(\) expects string, int given#'
+            path: RecordBuilders/CustomVariables.php
+            count: 5
+        # This plugin supports Matomo 5 and 6, but the analysis only ever sees one of them, so the
+        # version guard reads as decided. It is load-bearing on the other line.
+        -
+            identifier: ternary.alwaysTrue
+            path: RecordBuilders/CustomVariables.php
+            count: 1
+    bootstrapFiles:
+        - ../../bootstrap-phpstan.php
+    universalObjectCratesClasses:
+        - Piwik\Config
+        - Piwik\View
+        - Piwik\ViewDataTable\Config
+    scanDirectories:
+        - ../../