diff --git a/.github/workflows/ci-php-oldest.yml b/.github/workflows/ci-php-oldest.yml
index 932f6a4..f1d6353 100644
--- a/.github/workflows/ci-php-oldest.yml
+++ b/.github/workflows/ci-php-oldest.yml
@@ -8,13 +8,20 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '7.1'
       - uses: actions/cache@v3
         with:
           path: vendor
-          key: ${{ runner.OS }}-build-${{ hashFiles('**/composer.lock') }}
+          key: ${{ runner.OS }}-build-oldest-${{ hashFiles('**/composer.lock') }}
       - uses: php-actions/composer@v6
+        env:
+          COMPOSER: "composer-php-oldest.json"
         with:
           php_version: "7.1"
+          version: 1
       - name: PHP Lint
         run: vendor/bin/phplint --exclude=vendor
       - name: PHP Unit
diff --git a/composer-php-oldest.json b/composer-php-oldest.json
new file mode 100644
index 0000000..328940c
--- /dev/null
+++ b/composer-php-oldest.json
@@ -0,0 +1,21 @@
+{
+    "name": "qualityunit/tnef-decoder",
+    "description": "TNEF Decoder library extracted from squirell mail plugin to decode winmail.dat file in email attachment",
+    "keywords": ["tnef decoder", "winmail.dat"],
+    "license": "GPL-2.0-or-later",
+    "type": "library",
+    "homepage": "https://github.com/QualityUnit/TNEFDecoder",
+    "require": {
+		"php": ">=7.0",
+        "ext-mbstring": "*"
+    },
+    "require-dev": {
+        "overtrue/phplint": "3.2.0",
+        "phpunit/phpunit": "7.5.20"
+    },
+    "autoload": {
+        "psr-4": {
+            "TNEFDecoder\\": "src/TNEFDecoder/"
+        }
+    }
+}
diff --git a/src/TNEFDecoder/TNEFBuffer.php b/src/TNEFDecoder/TNEFBuffer.php
index 5b2eb41..5c8cc4d 100644
--- a/src/TNEFDecoder/TNEFBuffer.php
+++ b/src/TNEFDecoder/TNEFBuffer.php
@@ -1,4 +1,6 @@
-<?php namespace TNEFDecoder;
+<?php
+
+namespace TNEFDecoder;
 
 /**
   * SquirrelMail TNEF Decoder Plugin
@@ -16,8 +18,8 @@
 
 class TNEFBuffer
 {
-   private string $data;
-   private int $offset;
+   private $data;
+   private $offset;
 
    function __construct(string $data)
    {
@@ -25,7 +27,7 @@ function __construct(string $data)
       $this->offset = 0;
    }
 
-   function getBytes(int $numBytes): ?string
+   function getBytes(int $numBytes)
    {
       if ($this->getRemainingBytes() < $numBytes) {
          $this->offset = strlen($this->data);