diff --git a/system/CodeIgniter.php b/system/CodeIgniter.php
index 304c5d06371b..76325987d25f 100644
--- a/system/CodeIgniter.php
+++ b/system/CodeIgniter.php
@@ -186,11 +186,6 @@ public function __construct(App $config)
      */
     public function initialize()
     {
-        // Run this check for manual installations
-        if (! is_file(COMPOSER_PATH)) {
-            $this->resolvePlatformExtensions(); // @codeCoverageIgnore
-        }
-
         // Set default locale on the server
         Locale::setDefault($this->config->defaultLocale ?? 'en');
 
@@ -206,6 +201,8 @@ public function initialize()
      * @throws FrameworkException
      *
      * @codeCoverageIgnore
+     *
+     * @deprecated 4.5.0 Moved to system/bootstrap.php.
      */
     protected function resolvePlatformExtensions()
     {
diff --git a/system/bootstrap.php b/system/bootstrap.php
index 2c858d8bbc1c..a3f2ad0fba73 100644
--- a/system/bootstrap.php
+++ b/system/bootstrap.php
@@ -11,6 +11,7 @@
  * the LICENSE file that was distributed with this source code.
  */
 
+use CodeIgniter\Exceptions\FrameworkException;
 use Config\Autoload;
 use Config\Modules;
 use Config\Paths;
@@ -96,6 +97,33 @@
     require_once APPPATH . 'Config/Constants.php';
 }
 
+/*
+ * ---------------------------------------------------------------
+ * CHECK SYSTEM FOR MISSING REQUIRED PHP EXTENSIONS
+ * ---------------------------------------------------------------
+ */
+
+// Run this check for manual installations
+if (! is_file(COMPOSER_PATH)) {
+    $requiredExtensions = [
+        'intl',
+        'json',
+        'mbstring',
+    ];
+
+    $missingExtensions = [];
+
+    foreach ($requiredExtensions as $extension) {
+        if (! extension_loaded($extension)) {
+            $missingExtensions[] = $extension;
+        }
+    }
+
+    if ($missingExtensions !== []) {
+        throw FrameworkException::forMissingExtension(implode(', ', $missingExtensions));
+    }
+}
+
 /*
  * ---------------------------------------------------------------
  * LOAD COMMON FUNCTIONS