diff --git a/src/Context/CashContext.php b/src/Context/CashContext.php
index eb953e5..f8d6e46 100644
--- a/src/Context/CashContext.php
+++ b/src/Context/CashContext.php
@@ -21,7 +21,7 @@ final class CashContext implements Context
      *
      * For example, step 5 on CHF would allow CHF 0.00, CHF 0.05, CHF 0.10, etc.
      */
-    private int $step;
+    private readonly int $step;
 
     /**
      * @param int $step The cash rounding step, in minor units. Must be a multiple of 2 and/or 5.
diff --git a/src/Context/CustomContext.php b/src/Context/CustomContext.php
index 9af9cd9..b7f545d 100644
--- a/src/Context/CustomContext.php
+++ b/src/Context/CustomContext.php
@@ -18,14 +18,14 @@ final class CustomContext implements Context
     /**
      * The scale of the monies using this context.
      */
-    private int $scale;
+    private readonly int $scale;
 
     /**
      * An optional cash rounding step. Must be a multiple of 2 and/or 5.
      *
      * For example, scale=4 and step=5 would allow amounts of 0.0000, 0.0005, 0.0010, etc.
      */
-    private int $step;
+    private readonly int $step;
 
     /**
      * @param int $scale The scale of the monies using this context.
diff --git a/src/Currency.php b/src/Currency.php
index 18d99f9..c8c6b1d 100644
--- a/src/Currency.php
+++ b/src/Currency.php
@@ -20,7 +20,7 @@ final class Currency implements Stringable, JsonSerializable
      * For non ISO currencies no constraints are defined, but the code must be unique across an application, and must
      * not conflict with ISO currency codes.
      */
-    private string $currencyCode;
+    private readonly string $currencyCode;
 
     /**
      * The numeric currency code.
@@ -33,7 +33,7 @@ final class Currency implements Stringable, JsonSerializable
      *
      * The numeric code can be useful when storing monies in a database.
      */
-    private int $numericCode;
+    private readonly int $numericCode;
 
     /**
      * The name of the currency.
@@ -41,7 +41,7 @@ final class Currency implements Stringable, JsonSerializable
      * For ISO currencies this will be the official English name of the currency.
      * For non ISO currencies no constraints are defined.
      */
-    private string $name;
+    private readonly string $name;
 
     /**
      * The default number of fraction digits (typical scale) used with this currency.
@@ -49,7 +49,7 @@ final class Currency implements Stringable, JsonSerializable
      * For example, the default number of fraction digits for the Euro is 2, while for the Japanese Yen it is 0.
      * This cannot be a negative number.
      */
-    private int $defaultFractionDigits;
+    private readonly int $defaultFractionDigits;
 
     /**
      * Class constructor.
diff --git a/src/CurrencyConverter.php b/src/CurrencyConverter.php
index 67be717..d02c989 100644
--- a/src/CurrencyConverter.php
+++ b/src/CurrencyConverter.php
@@ -19,7 +19,7 @@ final class CurrencyConverter
     /**
      * The exchange rate provider.
      */
-    private ExchangeRateProvider $exchangeRateProvider;
+    private readonly ExchangeRateProvider $exchangeRateProvider;
 
     /**
      * @param ExchangeRateProvider $exchangeRateProvider The exchange rate provider.
diff --git a/src/Exception/CurrencyConversionException.php b/src/Exception/CurrencyConversionException.php
index e9e0cf2..45ad2dc 100644
--- a/src/Exception/CurrencyConversionException.php
+++ b/src/Exception/CurrencyConversionException.php
@@ -9,9 +9,9 @@
  */
 class CurrencyConversionException extends MoneyException
 {
-    private string $sourceCurrencyCode;
+    private readonly string $sourceCurrencyCode;
 
-    private string $targetCurrencyCode;
+    private readonly string $targetCurrencyCode;
 
     /**
      * CurrencyConversionException constructor.
diff --git a/src/ExchangeRateProvider/BaseCurrencyProvider.php b/src/ExchangeRateProvider/BaseCurrencyProvider.php
index 5ee7b84..0e6078e 100644
--- a/src/ExchangeRateProvider/BaseCurrencyProvider.php
+++ b/src/ExchangeRateProvider/BaseCurrencyProvider.php
@@ -23,12 +23,12 @@ final class BaseCurrencyProvider implements ExchangeRateProvider
     /**
      * The provider for rates relative to the base currency.
      */
-    private ExchangeRateProvider $provider;
+    private readonly ExchangeRateProvider $provider;
 
     /**
      * The code of the currency all the exchanges rates are based on.
      */
-    private string $baseCurrencyCode;
+    private readonly string $baseCurrencyCode;
 
     /**
      * @param ExchangeRateProvider $provider         The provider for rates relative to the base currency.
diff --git a/src/ExchangeRateProvider/CachedProvider.php b/src/ExchangeRateProvider/CachedProvider.php
index 4bae7c9..a08d44d 100644
--- a/src/ExchangeRateProvider/CachedProvider.php
+++ b/src/ExchangeRateProvider/CachedProvider.php
@@ -15,7 +15,7 @@ final class CachedProvider implements ExchangeRateProvider
     /**
      * The underlying exchange rate provider.
      */
-    private ExchangeRateProvider $provider;
+    private readonly ExchangeRateProvider $provider;
 
     /**
      * The cached exchange rates.
diff --git a/src/ExchangeRateProvider/PDOProvider.php b/src/ExchangeRateProvider/PDOProvider.php
index 544946a..007d521 100644
--- a/src/ExchangeRateProvider/PDOProvider.php
+++ b/src/ExchangeRateProvider/PDOProvider.php
@@ -16,17 +16,17 @@ final class PDOProvider implements ExchangeRateProvider
     /**
      * The SELECT statement.
      */
-    private \PDOStatement $statement;
+    private readonly \PDOStatement $statement;
 
     /**
      * The source currency code if fixed, or null if dynamic.
      */
-    private ?string $sourceCurrencyCode = null;
+    private readonly ?string $sourceCurrencyCode;
 
     /**
      * The target currency code if fixed, or null if dynamic.
      */
-    private ?string $targetCurrencyCode = null;
+    private readonly ?string $targetCurrencyCode;
 
     /**
      * Extra parameters set dynamically to resolve the query placeholders.
@@ -47,18 +47,24 @@ public function __construct(\PDO $pdo, PDOProviderConfiguration $configuration)
             $conditions[] = sprintf('(%s)', $configuration->whereConditions);
         }
 
+        $sourceCurrencyCode = null;
+        $targetCurrencyCode = null;
+
         if ($configuration->sourceCurrencyCode !== null) {
-            $this->sourceCurrencyCode = $configuration->sourceCurrencyCode;
+            $sourceCurrencyCode = $configuration->sourceCurrencyCode;
         } elseif ($configuration->sourceCurrencyColumnName !== null) {
             $conditions[] = sprintf('%s = ?', $configuration->sourceCurrencyColumnName);
         }
 
         if ($configuration->targetCurrencyCode !== null) {
-            $this->targetCurrencyCode = $configuration->targetCurrencyCode;
+            $targetCurrencyCode = $configuration->targetCurrencyCode;
         } elseif ($configuration->targetCurrencyColumnName !== null) {
             $conditions[] = sprintf('%s = ?', $configuration->targetCurrencyColumnName);
         }
 
+        $this->sourceCurrencyCode = $sourceCurrencyCode;
+        $this->targetCurrencyCode = $targetCurrencyCode;
+
         $conditions = implode(' AND ' , $conditions);
 
         $query = sprintf(
diff --git a/src/ISOCurrencyProvider.php b/src/ISOCurrencyProvider.php
index 4c43b63..c808f88 100644
--- a/src/ISOCurrencyProvider.php
+++ b/src/ISOCurrencyProvider.php
@@ -18,7 +18,7 @@ final class ISOCurrencyProvider
      *
      * @psalm-var array<string, array{string, int, string, int}>
      */
-    private array $currencyData;
+    private readonly array $currencyData;
 
     /**
      * An associative array of currency numeric code to currency code.
diff --git a/src/Money.php b/src/Money.php
index 97b469f..23cc02c 100644
--- a/src/Money.php
+++ b/src/Money.php
@@ -36,17 +36,17 @@ final class Money extends AbstractMoney
     /**
      * The amount.
      */
-    private BigDecimal $amount;
+    private readonly BigDecimal $amount;
 
     /**
      * The currency.
      */
-    private Currency $currency;
+    private readonly Currency $currency;
 
     /**
      * The context that defines the capability of this Money.
      */
-    private Context $context;
+    private readonly Context $context;
 
     /**
      * @param BigDecimal $amount
diff --git a/src/MoneyComparator.php b/src/MoneyComparator.php
index 9c6354d..6fbb0f9 100644
--- a/src/MoneyComparator.php
+++ b/src/MoneyComparator.php
@@ -21,7 +21,7 @@ final class MoneyComparator
     /**
      * The exchange rate provider.
      */
-    private ExchangeRateProvider $exchangeRateProvider;
+    private readonly ExchangeRateProvider $exchangeRateProvider;
 
     /**
      * Class constructor.
diff --git a/src/RationalMoney.php b/src/RationalMoney.php
index fa98d39..5d7ef67 100644
--- a/src/RationalMoney.php
+++ b/src/RationalMoney.php
@@ -18,9 +18,9 @@
  */
 final class RationalMoney extends AbstractMoney
 {
-    private BigRational $amount;
+    private readonly BigRational $amount;
 
-    private Currency $currency;
+    private readonly Currency $currency;
 
     /**
      * Class constructor.