diff --git a/administrator/includes/app.php b/administrator/includes/app.php
index 161abb4a2f85e..0fdbb854aaed3 100644
--- a/administrator/includes/app.php
+++ b/administrator/includes/app.php
@@ -17,14 +17,7 @@
     include_once dirname(__DIR__) . '/defines.php';
 }
 
-if (!defined('_JDEFINES')) {
-    define('JPATH_BASE', dirname(__DIR__));
-    require_once JPATH_BASE . '/includes/defines.php';
-}
-
-if (!defined('JPATH_PUBLIC')) {
-    define('JPATH_PUBLIC', JPATH_ROOT);
-}
+require_once __DIR__ . '/defines.php';
 
 // Check for presence of vendor dependencies not included in the git repository
 if (!file_exists(JPATH_LIBRARIES . '/vendor/autoload.php') || !is_dir(JPATH_PUBLIC . '/media/vendor')) {
diff --git a/administrator/includes/defines.php b/administrator/includes/defines.php
index ddf7c93e779e5..23b7ff2790212 100644
--- a/administrator/includes/defines.php
+++ b/administrator/includes/defines.php
@@ -9,21 +9,24 @@
 
 defined('_JEXEC') or die;
 
+// Define JPATH constants if not defined yet
+defined('JPATH_BASE') || define('JPATH_BASE', dirname(__DIR__));
+
 // Global definitions
 $parts = explode(DIRECTORY_SEPARATOR, JPATH_BASE);
 array_pop($parts);
 
 // Defines
-define('JPATH_ROOT', implode(DIRECTORY_SEPARATOR, $parts));
-define('JPATH_SITE', JPATH_ROOT);
-define('JPATH_PUBLIC', JPATH_ROOT);
-define('JPATH_CONFIGURATION', JPATH_ROOT);
-define('JPATH_ADMINISTRATOR', JPATH_ROOT . DIRECTORY_SEPARATOR . 'administrator');
-define('JPATH_LIBRARIES', JPATH_ROOT . DIRECTORY_SEPARATOR . 'libraries');
-define('JPATH_PLUGINS', JPATH_ROOT . DIRECTORY_SEPARATOR . 'plugins');
-define('JPATH_INSTALLATION', JPATH_ROOT . DIRECTORY_SEPARATOR . 'installation');
-define('JPATH_THEMES', JPATH_BASE . DIRECTORY_SEPARATOR . 'templates');
-define('JPATH_CACHE', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'cache');
-define('JPATH_MANIFESTS', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'manifests');
-define('JPATH_API', JPATH_ROOT . DIRECTORY_SEPARATOR . 'api');
-define('JPATH_CLI', JPATH_ROOT . DIRECTORY_SEPARATOR . 'cli');
+defined('JPATH_ROOT') || define('JPATH_ROOT', implode(DIRECTORY_SEPARATOR, $parts));
+defined('JPATH_SITE') || define('JPATH_SITE', JPATH_ROOT);
+defined('JPATH_PUBLIC') || define('JPATH_PUBLIC', JPATH_ROOT);
+defined('JPATH_CONFIGURATION') || define('JPATH_CONFIGURATION', JPATH_ROOT);
+defined('JPATH_ADMINISTRATOR') || define('JPATH_ADMINISTRATOR', JPATH_ROOT . DIRECTORY_SEPARATOR . 'administrator');
+defined('JPATH_LIBRARIES') || define('JPATH_LIBRARIES', JPATH_ROOT . DIRECTORY_SEPARATOR . 'libraries');
+defined('JPATH_PLUGINS') || define('JPATH_PLUGINS', JPATH_ROOT . DIRECTORY_SEPARATOR . 'plugins');
+defined('JPATH_INSTALLATION') || define('JPATH_INSTALLATION', JPATH_ROOT . DIRECTORY_SEPARATOR . 'installation');
+defined('JPATH_THEMES') || define('JPATH_THEMES', JPATH_BASE . DIRECTORY_SEPARATOR . 'templates');
+defined('JPATH_CACHE') || define('JPATH_CACHE', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'cache');
+defined('JPATH_MANIFESTS') || define('JPATH_MANIFESTS', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'manifests');
+defined('JPATH_API') || define('JPATH_API', JPATH_ROOT . DIRECTORY_SEPARATOR . 'api');
+defined('JPATH_CLI') || define('JPATH_CLI', JPATH_ROOT . DIRECTORY_SEPARATOR . 'cli');
diff --git a/api/includes/app.php b/api/includes/app.php
index d55fc47a3c6cd..61559562e1469 100644
--- a/api/includes/app.php
+++ b/api/includes/app.php
@@ -17,14 +17,7 @@
     include_once dirname(__DIR__) . '/defines.php';
 }
 
-if (!defined('_JDEFINES')) {
-    define('JPATH_BASE', dirname(__DIR__));
-    require_once JPATH_BASE . '/includes/defines.php';
-}
-
-if (!defined('JPATH_PUBLIC')) {
-    define('JPATH_PUBLIC', JPATH_ROOT);
-}
+require_once __DIR__ . '/defines.php';
 
 require_once JPATH_BASE . '/includes/framework.php';
 
diff --git a/api/includes/defines.php b/api/includes/defines.php
index 09df2661f67d3..bc05a4ac58f31 100644
--- a/api/includes/defines.php
+++ b/api/includes/defines.php
@@ -9,21 +9,24 @@
 
 defined('_JEXEC') or die;
 
+// Define JPATH constants if not defined yet
+defined('JPATH_BASE') || define('JPATH_BASE', dirname(__DIR__));
+
 // Global definitions
 $parts = explode(DIRECTORY_SEPARATOR, JPATH_BASE);
 array_pop($parts);
 
-// Defines.
-define('JPATH_ROOT', implode(DIRECTORY_SEPARATOR, $parts));
-define('JPATH_SITE', JPATH_ROOT);
-define('JPATH_PUBLIC', JPATH_ROOT);
-define('JPATH_CONFIGURATION', JPATH_ROOT);
-define('JPATH_ADMINISTRATOR', JPATH_ROOT . DIRECTORY_SEPARATOR . 'administrator');
-define('JPATH_LIBRARIES', JPATH_ROOT . DIRECTORY_SEPARATOR . 'libraries');
-define('JPATH_PLUGINS', JPATH_ROOT . DIRECTORY_SEPARATOR . 'plugins');
-define('JPATH_INSTALLATION', JPATH_ROOT . DIRECTORY_SEPARATOR . 'installation');
-define('JPATH_THEMES', JPATH_BASE . DIRECTORY_SEPARATOR . 'templates');
-define('JPATH_CACHE', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'cache');
-define('JPATH_MANIFESTS', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'manifests');
-define('JPATH_API', JPATH_ROOT . DIRECTORY_SEPARATOR . 'api');
-define('JPATH_CLI', JPATH_ROOT . DIRECTORY_SEPARATOR . 'cli');
+// Defines
+defined('JPATH_ROOT') || define('JPATH_ROOT', implode(DIRECTORY_SEPARATOR, $parts));
+defined('JPATH_SITE') || define('JPATH_SITE', JPATH_ROOT);
+defined('JPATH_PUBLIC') || define('JPATH_PUBLIC', JPATH_ROOT);
+defined('JPATH_CONFIGURATION') || define('JPATH_CONFIGURATION', JPATH_ROOT);
+defined('JPATH_ADMINISTRATOR') || define('JPATH_ADMINISTRATOR', JPATH_ROOT . DIRECTORY_SEPARATOR . 'administrator');
+defined('JPATH_LIBRARIES') || define('JPATH_LIBRARIES', JPATH_ROOT . DIRECTORY_SEPARATOR . 'libraries');
+defined('JPATH_PLUGINS') || define('JPATH_PLUGINS', JPATH_ROOT . DIRECTORY_SEPARATOR . 'plugins');
+defined('JPATH_INSTALLATION') || define('JPATH_INSTALLATION', JPATH_ROOT . DIRECTORY_SEPARATOR . 'installation');
+defined('JPATH_THEMES') || define('JPATH_THEMES', JPATH_BASE . DIRECTORY_SEPARATOR . 'templates');
+defined('JPATH_CACHE') || define('JPATH_CACHE', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'cache');
+defined('JPATH_MANIFESTS') || define('JPATH_MANIFESTS', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'manifests');
+defined('JPATH_API') || define('JPATH_API', JPATH_ROOT . DIRECTORY_SEPARATOR . 'api');
+defined('JPATH_CLI') || define('JPATH_CLI', JPATH_ROOT . DIRECTORY_SEPARATOR . 'cli');
diff --git a/includes/app.php b/includes/app.php
index 53a25b9c8f0f4..7a34a062e99f3 100644
--- a/includes/app.php
+++ b/includes/app.php
@@ -17,10 +17,7 @@
     include_once dirname(__DIR__) . '/defines.php';
 }
 
-if (!defined('_JDEFINES')) {
-    define('JPATH_BASE', dirname(__DIR__));
-    require_once JPATH_BASE . '/includes/defines.php';
-}
+require_once __DIR__ . '/defines.php';
 
 if (!defined('JPATH_PUBLIC')) {
     define('JPATH_PUBLIC', JPATH_ROOT);
diff --git a/includes/defines.php b/includes/defines.php
index 179cb9b9d89a2..cf156324c5cb6 100644
--- a/includes/defines.php
+++ b/includes/defines.php
@@ -9,20 +9,18 @@
 
 defined('_JEXEC') or die;
 
-// Global definitions
-$parts = explode(DIRECTORY_SEPARATOR, JPATH_BASE);
-
-// Defines.
-define('JPATH_ROOT', implode(DIRECTORY_SEPARATOR, $parts));
-define('JPATH_SITE', JPATH_ROOT);
-define('JPATH_CONFIGURATION', JPATH_ROOT);
-define('JPATH_PUBLIC', JPATH_ROOT);
-define('JPATH_ADMINISTRATOR', JPATH_ROOT . DIRECTORY_SEPARATOR . 'administrator');
-define('JPATH_LIBRARIES', JPATH_ROOT . DIRECTORY_SEPARATOR . 'libraries');
-define('JPATH_PLUGINS', JPATH_ROOT . DIRECTORY_SEPARATOR . 'plugins');
-define('JPATH_INSTALLATION', JPATH_ROOT . DIRECTORY_SEPARATOR . 'installation');
-define('JPATH_THEMES', JPATH_BASE . DIRECTORY_SEPARATOR . 'templates');
-define('JPATH_CACHE', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'cache');
-define('JPATH_MANIFESTS', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'manifests');
-define('JPATH_API', JPATH_ROOT . DIRECTORY_SEPARATOR . 'api');
-define('JPATH_CLI', JPATH_ROOT . DIRECTORY_SEPARATOR . 'cli');
+// Define JPATH constants if not defined yet
+defined('JPATH_BASE') || define('JPATH_BASE', dirname(__DIR__));
+defined('JPATH_ROOT') || define('JPATH_ROOT', JPATH_BASE);
+defined('JPATH_SITE') || define('JPATH_SITE', JPATH_ROOT);
+defined('JPATH_PUBLIC') || define('JPATH_PUBLIC', JPATH_ROOT);
+defined('JPATH_CONFIGURATION') || define('JPATH_CONFIGURATION', JPATH_ROOT);
+defined('JPATH_ADMINISTRATOR') || define('JPATH_ADMINISTRATOR', JPATH_ROOT . DIRECTORY_SEPARATOR . 'administrator');
+defined('JPATH_LIBRARIES') || define('JPATH_LIBRARIES', JPATH_ROOT . DIRECTORY_SEPARATOR . 'libraries');
+defined('JPATH_PLUGINS') || define('JPATH_PLUGINS', JPATH_ROOT . DIRECTORY_SEPARATOR . 'plugins');
+defined('JPATH_INSTALLATION') || define('JPATH_INSTALLATION', JPATH_ROOT . DIRECTORY_SEPARATOR . 'installation');
+defined('JPATH_THEMES') || define('JPATH_THEMES', JPATH_BASE . DIRECTORY_SEPARATOR . 'templates');
+defined('JPATH_CACHE') || define('JPATH_CACHE', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'cache');
+defined('JPATH_MANIFESTS') || define('JPATH_MANIFESTS', JPATH_ADMINISTRATOR . DIRECTORY_SEPARATOR . 'manifests');
+defined('JPATH_API') || define('JPATH_API', JPATH_ROOT . DIRECTORY_SEPARATOR . 'api');
+defined('JPATH_CLI') || define('JPATH_CLI', JPATH_ROOT . DIRECTORY_SEPARATOR . 'cli');
diff --git a/libraries/src/Router/SiteRouter.php b/libraries/src/Router/SiteRouter.php
index 73edd157c90f3..49e6e13e3f600 100644
--- a/libraries/src/Router/SiteRouter.php
+++ b/libraries/src/Router/SiteRouter.php
@@ -149,11 +149,11 @@ public function parseInit(&$router, &$uri)
         if (preg_match("#.*?\.php#u", $path, $matches)) {
             // Get the current entry point path relative to the site path.
             $scriptPath         = realpath($_SERVER['SCRIPT_FILENAME'] ?: str_replace('\\\\', '\\', $_SERVER['PATH_TRANSLATED']));
-            $relativeScriptPath = str_replace('\\', '/', str_replace(JPATH_SITE, '', $scriptPath));
+            $relativeScriptPath = str_replace('\\', '/', str_replace(JPATH_PUBLIC, '', $scriptPath));
 
             // If a php file has been found in the request path, check to see if it is a valid file.
             // Also verify that it represents the same file from the server variable for entry script.
-            if (is_file(JPATH_SITE . $matches[0]) && ($matches[0] === $relativeScriptPath)) {
+            if (is_file(JPATH_PUBLIC . $matches[0]) && ($matches[0] === $relativeScriptPath)) {
                 // Remove the entry point segments from the request path for proper routing.
                 $path = str_replace($matches[0], '', $path);
             }