diff --git a/CHANGELOG.md b/CHANGELOG.md
index c3cd6bd..2de8e7c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [3.5.5] - 2025-10-09
+### Fixes
+- Fix deprecation warnings on PHP 8.4
+- Restrict supported PHP versions to 7.1 and above.
+
 ## [3.5.4] - 2025-09-12
 ### Added
 - Provide `setFormTemplate` method to set `form_template` for the `createPaymentRequest`.
diff --git a/README.md b/README.md
index bad80c2..c248c36 100644
--- a/README.md
+++ b/README.md
@@ -26,7 +26,7 @@ try {
 
 ## Requirements
 
-The AltaPay API PHP requires PHP 7.0.0 or greater with the following extensions installed:
+The AltaPay API PHP requires PHP 7.1.0 or greater with the following extensions installed:
 
 - date
 - filter
diff --git a/composer.json b/composer.json
index 464c6f7..3e2f6c2 100644
--- a/composer.json
+++ b/composer.json
@@ -22,7 +22,7 @@
         ]
     },
     "require": {
-        "php": "^7.0 || ^8.0",
+        "php": "^7.1 || ^8.0",
         "ext-date": "*",
         "ext-filter": "*",
         "ext-mbstring": "*",
diff --git a/src/AbstractApi.php b/src/AbstractApi.php
index d5eba3f..0b66785 100644
--- a/src/AbstractApi.php
+++ b/src/AbstractApi.php
@@ -55,7 +55,7 @@ abstract class AbstractApi
     /**
      * PHP API version
      */
-    const PHP_API_VERSION = '3.5.3';
+    const PHP_API_VERSION = '3.5.5';
 
     /**
      * Event dispatcher
diff --git a/src/Factory.php b/src/Factory.php
index f5b9899..49082c7 100644
--- a/src/Factory.php
+++ b/src/Factory.php
@@ -63,7 +63,7 @@ class Factory
      * @return T
      * @throws ClassDoesNotExistsException
      */
-    public static function create($class, Authentication $authentication = null)
+    public static function create($class, ?Authentication $authentication = null)
     {
         if (class_exists($class)) {
             return new $class($authentication);
diff --git a/src/Request/Customer.php b/src/Request/Customer.php
index f04cb8e..2b98747 100644
--- a/src/Request/Customer.php
+++ b/src/Request/Customer.php
@@ -267,7 +267,7 @@ class Customer extends AbstractSerializer
      *
      * @param Address|null $billingAddress Billing address
      */
-    public function __construct(Address $billingAddress = null)
+    public function __construct(?Address $billingAddress = null)
     {
         $this->billing = $billingAddress;
     }
diff --git a/src/Response/AbstractResponse.php b/src/Response/AbstractResponse.php
index c15783c..d6fac30 100644
--- a/src/Response/AbstractResponse.php
+++ b/src/Response/AbstractResponse.php
@@ -49,7 +49,7 @@ abstract class AbstractResponse
      * @return void
      * @throws \InvalidArgumentException
      */
-    public function headerSetter(\SimpleXMLElement $xml = null)
+    public function headerSetter(?\SimpleXMLElement $xml = null)
     {
         if ($xml) {
             $this->Header = ResponseSerializer::serialize(Header::class, $xml);
@@ -64,7 +64,7 @@ public function headerSetter(\SimpleXMLElement $xml = null)
      * @return static
      * @throws \InvalidArgumentException
      */
-    public function deserialize(\SimpleXMLElement $xml = null)
+    public function deserialize(?\SimpleXMLElement $xml = null)
     {
         $object = clone $this;
 
diff --git a/src/Serializer/ResponseSerializer.php b/src/Serializer/ResponseSerializer.php
index 7403e27..ecb0908 100644
--- a/src/Serializer/ResponseSerializer.php
+++ b/src/Serializer/ResponseSerializer.php
@@ -45,7 +45,7 @@ class ResponseSerializer
     public static function serialize(
         $objectName,
         \SimpleXMLElement $data,
-        \SimpleXMLElement $header = null
+        ?\SimpleXMLElement $header = null
     ) {
         $object = new $objectName();
         $object->headerSetter($header);