diff --git a/.github/workflows/deploy-userguide-latest.yml b/.github/workflows/deploy-userguide-latest.yml
index ab366268e5ee..69d9b2903c84 100644
--- a/.github/workflows/deploy-userguide-latest.yml
+++ b/.github/workflows/deploy-userguide-latest.yml
@@ -30,7 +30,7 @@ jobs:
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '8.0'
+          php-version: '8.1'
           coverage: none
 
       # Build the latest User Guide
diff --git a/.github/workflows/reusable-phpunit-test.yml b/.github/workflows/reusable-phpunit-test.yml
index 0963f5865242..3f75e2e73043 100644
--- a/.github/workflows/reusable-phpunit-test.yml
+++ b/.github/workflows/reusable-phpunit-test.yml
@@ -66,7 +66,7 @@ jobs:
     # Service containers cannot be extracted to caller workflows yet
     services:
       mysql:
-        image: mysql:${{ inputs.mysql-version || '8.0' }}
+        image: mysql:${{ inputs.mysql-version || '8.1' }}
         env:
           MYSQL_ALLOW_EMPTY_PASSWORD: yes
           MYSQL_DATABASE: test
diff --git a/.github/workflows/test-coding-standards.yml b/.github/workflows/test-coding-standards.yml
index cd79e13b243e..733bbc570e5a 100644
--- a/.github/workflows/test-coding-standards.yml
+++ b/.github/workflows/test-coding-standards.yml
@@ -28,7 +28,7 @@ jobs:
       fail-fast: false
       matrix:
         php-version:
-          - '8.0'
+          - '8.1'
           - '8.2'
 
     steps:
diff --git a/.github/workflows/test-phpcpd.yml b/.github/workflows/test-phpcpd.yml
index 53daa8ebab95..146fd31bb7b2 100644
--- a/.github/workflows/test-phpcpd.yml
+++ b/.github/workflows/test-phpcpd.yml
@@ -41,7 +41,7 @@ jobs:
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '8.0'
+          php-version: '8.1'
           tools: phpcpd
           extensions: dom, mbstring
 
diff --git a/.github/workflows/test-phpunit.yml b/.github/workflows/test-phpunit.yml
index 9beee1b3aa09..349504b51e86 100644
--- a/.github/workflows/test-phpunit.yml
+++ b/.github/workflows/test-phpunit.yml
@@ -55,7 +55,6 @@ jobs:
     strategy:
       matrix:
         php-version:
-          - '8.0'
           - '8.1'
           - '8.2'
           - '8.3'
@@ -84,7 +83,6 @@ jobs:
       fail-fast: false
       matrix:
         php-version:
-          - '8.0'
           - '8.1'
           - '8.2'
           - '8.3'
@@ -97,7 +95,7 @@ jobs:
         mysql-version:
           - '5.7'
         include:
-          - php-version: '8.0'
+          - php-version: '8.1'
             db-platform: MySQLi
             mysql-version: '8.0'
           - php-version: '8.3'
@@ -125,7 +123,6 @@ jobs:
     strategy:
       matrix:
         php-version:
-          - '8.0'
           - '8.1'
           - '8.2'
           - '8.3'
@@ -153,7 +150,6 @@ jobs:
     strategy:
       matrix:
         php-version:
-          - '8.0'
           - '8.1'
           - '8.2'
           - '8.3'
diff --git a/.github/workflows/test-rector.yml b/.github/workflows/test-rector.yml
index e29ee6ba64e7..0c70dd91ecdc 100644
--- a/.github/workflows/test-rector.yml
+++ b/.github/workflows/test-rector.yml
@@ -45,7 +45,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-versions: ['8.0', '8.2']
+        php-versions: ['8.1', '8.3']
         paths:
           - app
           - system
diff --git a/README.md b/README.md
index d5f8ec34dd51..c109cc9f566f 100644
--- a/README.md
+++ b/README.md
@@ -86,7 +86,7 @@ Made with [contrib.rocks](https://contrib.rocks).
 
 ## Server Requirements
 
-PHP version 8.0 or higher is required, with the following extensions installed:
+PHP version 8.1 or higher is required, with the following extensions installed:
 
 - [intl](http://php.net/manual/en/intl.requirements.php)
 - [mbstring](http://php.net/manual/en/mbstring.installation.php)
diff --git a/admin/framework/README.md b/admin/framework/README.md
index f6f81cd65184..58a9f0a45b09 100644
--- a/admin/framework/README.md
+++ b/admin/framework/README.md
@@ -42,7 +42,7 @@ Please read the [*Contributing to CodeIgniter*](https://github.com/codeigniter4/
 
 ## Server Requirements
 
-PHP version 8.0 or higher is required, with the following extensions installed:
+PHP version 8.1 or higher is required, with the following extensions installed:
 
 - [intl](http://php.net/manual/en/intl.requirements.php)
 - [mbstring](http://php.net/manual/en/mbstring.installation.php)
diff --git a/admin/framework/composer.json b/admin/framework/composer.json
index 132932acc25d..44fbb67591c0 100644
--- a/admin/framework/composer.json
+++ b/admin/framework/composer.json
@@ -10,7 +10,7 @@
         "slack": "https://codeigniterchat.slack.com"
     },
     "require": {
-        "php": "^8.0",
+        "php": "^8.1",
         "ext-intl": "*",
         "ext-json": "*",
         "ext-mbstring": "*",
diff --git a/admin/starter/.github/workflows/phpunit.yml b/admin/starter/.github/workflows/phpunit.yml
index 0489a9376466..f43435df9765 100644
--- a/admin/starter/.github/workflows/phpunit.yml
+++ b/admin/starter/.github/workflows/phpunit.yml
@@ -11,7 +11,7 @@ jobs:
 
     strategy:
       matrix:
-        php-versions: ['8.0', '8.2']
+        php-versions: ['8.1', '8.3']
 
     runs-on: ubuntu-latest
 
diff --git a/admin/starter/README.md b/admin/starter/README.md
index fc8f69b5eb1d..5fc426b6c8f7 100644
--- a/admin/starter/README.md
+++ b/admin/starter/README.md
@@ -50,7 +50,7 @@ Problems with it can be raised on our forum, or as issues in the main repository
 
 ## Server Requirements
 
-PHP version 8.0 or higher is required, with the following extensions installed:
+PHP version 8.1 or higher is required, with the following extensions installed:
 
 - [intl](http://php.net/manual/en/intl.requirements.php)
 - [mbstring](http://php.net/manual/en/mbstring.installation.php)
diff --git a/admin/starter/composer.json b/admin/starter/composer.json
index 35b791c3f050..27b4efc5903b 100644
--- a/admin/starter/composer.json
+++ b/admin/starter/composer.json
@@ -10,7 +10,7 @@
         "slack": "https://codeigniterchat.slack.com"
     },
     "require": {
-        "php": "^8.0",
+        "php": "^8.1",
         "codeigniter4/framework": "^4.0"
     },
     "require-dev": {
diff --git a/composer.json b/composer.json
index c20c5a2e5b41..6510a6872599 100644
--- a/composer.json
+++ b/composer.json
@@ -10,7 +10,7 @@
         "slack": "https://codeigniterchat.slack.com"
     },
     "require": {
-        "php": "^8.0",
+        "php": "^8.1",
         "ext-intl": "*",
         "ext-json": "*",
         "ext-mbstring": "*",
diff --git a/contributing/pull_request.md b/contributing/pull_request.md
index 8e5a489bb519..6ac917c30041 100644
--- a/contributing/pull_request.md
+++ b/contributing/pull_request.md
@@ -136,7 +136,7 @@ See [Contribution CSS](./css.md).
 
 ### Compatibility
 
-CodeIgniter4 requires [PHP 8.0](https://php.net/releases/8_0_0.php).
+CodeIgniter4 requires [PHP 8.1](https://php.net/releases/8_1_0.php).
 
 ### Backwards Compatibility
 
diff --git a/public/index.php b/public/index.php
index 826966179eec..8cf5ce347c82 100644
--- a/public/index.php
+++ b/public/index.php
@@ -1,7 +1,7 @@
 <?php
 
 // Check PHP version.
-$minPhpVersion = '8.0'; // If you update this, don't forget to update `spark`.
+$minPhpVersion = '8.1'; // If you update this, don't forget to update `spark`.
 if (version_compare(PHP_VERSION, $minPhpVersion, '<')) {
     $message = sprintf(
         'Your PHP version must be %s or higher to run CodeIgniter. Current version: %s',
diff --git a/spark b/spark
index 22ff16b82f26..8e71c7444080 100755
--- a/spark
+++ b/spark
@@ -27,7 +27,7 @@ if (strpos(PHP_SAPI, 'cgi') === 0) {
 }
 
 // Check PHP version.
-$minPhpVersion = '8.0'; // If you update this, don't forget to update `public/index.php`.
+$minPhpVersion = '8.1'; // If you update this, don't forget to update `public/index.php`.
 if (version_compare(PHP_VERSION, $minPhpVersion, '<')) {
     $message = sprintf(
         'Your PHP version must be %s or higher to run CodeIgniter. Current version: %s',
diff --git a/system/Cookie/Cookie.php b/system/Cookie/Cookie.php
index 69e40017bb65..68a748cb30eb 100644
--- a/system/Cookie/Cookie.php
+++ b/system/Cookie/Cookie.php
@@ -633,7 +633,7 @@ public function toHeaderString(): string
     /**
      * {@inheritDoc}
      */
-    public function __toString()
+    public function __toString(): string
     {
         $cookieHeader = [];
 
diff --git a/system/View/Cells/Cell.php b/system/View/Cells/Cell.php
index dc97fd996544..328bae1f8953 100644
--- a/system/View/Cells/Cell.php
+++ b/system/View/Cells/Cell.php
@@ -118,7 +118,7 @@ final protected function view(?string $view, array $data = []): string
     /**
      * Provides capability to render on string casting.
      */
-    public function __toString()
+    public function __toString(): string
     {
         return $this->render();
     }
diff --git a/user_guide_src/source/changelogs/v4.5.0.rst b/user_guide_src/source/changelogs/v4.5.0.rst
index 89910fa84f3d..e7587a312a39 100644
--- a/user_guide_src/source/changelogs/v4.5.0.rst
+++ b/user_guide_src/source/changelogs/v4.5.0.rst
@@ -12,7 +12,7 @@ Release Date: Unreleased
 Highlights
 **********
 
-- Update minimal PHP requirement to 8.0.
+- Update minimal PHP requirement to 8.1.
 - TBD
 
 BREAKING
diff --git a/user_guide_src/source/installation/installing_composer.rst b/user_guide_src/source/installation/installing_composer.rst
index c3f8bec85fd7..5310b00963f8 100644
--- a/user_guide_src/source/installation/installing_composer.rst
+++ b/user_guide_src/source/installation/installing_composer.rst
@@ -142,7 +142,7 @@ edit **composer.json** manually.
 If you try the ``4.6`` branch, change the version to ``4.6.x-dev``::
 
     "require": {
-        "php": "^8.0",
+        "php": "^8.1",
         "codeigniter4/codeigniter4": "4.6.x-dev"
     },
 
diff --git a/user_guide_src/source/installation/upgrade_4xx.rst b/user_guide_src/source/installation/upgrade_4xx.rst
index f452fc247f2a..9f9fdd259b16 100644
--- a/user_guide_src/source/installation/upgrade_4xx.rst
+++ b/user_guide_src/source/installation/upgrade_4xx.rst
@@ -41,7 +41,7 @@ Downloads
 Namespaces
 ==========
 
-- CI4 is built for PHP 8.0+, and everything in the framework is namespaced,
+- CI4 is built for PHP 8.1+, and everything in the framework is namespaced,
   except for the helper and lang files.
 
 Application Structure
diff --git a/user_guide_src/source/intro/requirements.rst b/user_guide_src/source/intro/requirements.rst
index 17154a952563..607022bd82fe 100644
--- a/user_guide_src/source/intro/requirements.rst
+++ b/user_guide_src/source/intro/requirements.rst
@@ -10,7 +10,7 @@ Server Requirements
 PHP and Required Extensions
 ***************************
 
-`PHP <https://www.php.net/>`_ version 8.0 or newer is required, with the following PHP extensions are enabled:
+`PHP <https://www.php.net/>`_ version 8.1 or newer is required, with the following PHP extensions are enabled:
 
   - `intl <https://www.php.net/manual/en/intl.requirements.php>`_
   - `mbstring <https://www.php.net/manual/en/mbstring.requirements.php>`_