diff --git a/phpdotnet/phd/ErrorHandler.php b/phpdotnet/phd/ErrorHandler.php
index 202f4cf4..dc6061fe 100644
--- a/phpdotnet/phd/ErrorHandler.php
+++ b/phpdotnet/phd/ErrorHandler.php
@@ -20,7 +20,7 @@ class ErrorHandler
     private bool $recursive = false;
     
     public function __construct(
-        private OutputHandler $outputHandler
+        private readonly OutputHandler $outputHandler
     ) {}
 
     public function handleError($errno, $msg, $file, $line) {
diff --git a/phpdotnet/phd/IndexRepository.php b/phpdotnet/phd/IndexRepository.php
index 0ecf5dd1..2d17d8f4 100644
--- a/phpdotnet/phd/IndexRepository.php
+++ b/phpdotnet/phd/IndexRepository.php
@@ -11,7 +11,7 @@ class IndexRepository
     private array $examples = [];
 
     public function __construct(
-        private \SQLite3 $db
+        private readonly \SQLite3 $db
     ) {}
 
     public function init(): void {
diff --git a/phpdotnet/phd/Options/Handler.php b/phpdotnet/phd/Options/Handler.php
index a2656227..883cb6de 100644
--- a/phpdotnet/phd/Options/Handler.php
+++ b/phpdotnet/phd/Options/Handler.php
@@ -4,9 +4,9 @@
 class Options_Handler implements Options_Interface
 {
     public function __construct(
-        private Config $config,
-        private Format_Factory $formatFactory,
-        private OutputHandler $outputHandler
+        private readonly Config         $config,
+        private readonly Format_Factory $formatFactory,
+        private readonly OutputHandler  $outputHandler
     ) {}
 
     /**
diff --git a/phpdotnet/phd/OutputHandler.php b/phpdotnet/phd/OutputHandler.php
index 96da4c3e..3da09119 100644
--- a/phpdotnet/phd/OutputHandler.php
+++ b/phpdotnet/phd/OutputHandler.php
@@ -24,7 +24,7 @@ class OutputHandler
     ];
     
     public function __construct(
-        private Config $config
+        private readonly Config $config
     ) {}
 
     /**