diff --git a/composer.json b/composer.json
index 3d2454b..e9b4a75 100644
--- a/composer.json
+++ b/composer.json
@@ -24,7 +24,7 @@
         "sepia/po-parser": "^6.0",
         "sentry/sdk": "^3.3",
         "ext-pdo": "^7.3",
-        "ext-json": "^7.3",
+        "ext-json": "^1.7",
         "ext-yaml": "^2.0",
         "league/mime-type-detection": "^1.11"
     },
diff --git a/src/Date.php b/src/Date.php
index 9da0db2..a43401f 100644
--- a/src/Date.php
+++ b/src/Date.php
@@ -9,14 +9,14 @@ class Date {
    * @see strftime()
    * @var string
    */
-  public static string $date_format = "%d/%m/%Y";
+  public static $date_format = "%d/%m/%Y";
 
   /**
    * The datetime format
    * @see strftime()
    * @var string
    */
-  public static string $date_time_format = "%d/%m/%Y %H:%M:%S";
+  public static $date_time_format = "%d/%m/%Y %H:%M:%S";
 
   /**
    * Format a timestamp as date/datetime string
diff --git a/src/Db.php b/src/Db.php
index 3d5b895..b720ae3 100644
--- a/src/Db.php
+++ b/src/Db.php
@@ -24,13 +24,13 @@ class Db {
    * Date format as returned by database
    * @var string
    */
-  protected string $date_format = '%Y-%m-%d';
+  protected $date_format = '%Y-%m-%d';
 
   /**
    * Datetime format as returned by database
    * @var string
    */
-  protected string $datetime_format = '%Y-%m-%d %H:%M:%S';
+  protected $datetime_format = '%Y-%m-%d %H:%M:%S';
 
   /**
    * Locale for time (as expected by LC_TIME)
diff --git a/src/I18n.php b/src/I18n.php
index 892d504..66012e6 100644
--- a/src/I18n.php
+++ b/src/I18n.php
@@ -15,19 +15,19 @@ class I18n {
    * The core root directory path of translation files
    * @var string
    */
-  protected static string $core_root_path = __DIR__.'/../locales';
+  protected static $core_root_path = __DIR__.'/../locales';
 
   /**
    * The root directory path of translation files
    * @var string
    */
-  protected static string $root_path = 'locales';
+  protected static $root_path = 'locales';
 
   /**
    * The default locale
    * @var string
    */
-  protected static string $default_locale = 'en_US.UTF8';
+  protected static $default_locale = 'en_US.UTF8';
 
   /**
    * Initialize translation system
diff --git a/src/Log.php b/src/Log.php
index c2cc595..e2eb7ad 100644
--- a/src/Log.php
+++ b/src/Log.php
@@ -36,7 +36,7 @@ class Log {
    * Default log level
    * @var string
    */
-  protected static string $default_level = 'WARNING';
+  protected static $default_level = 'WARNING';
 
   /*
    * Current log level
diff --git a/src/SentryIntegration.php b/src/SentryIntegration.php
index 91bda43..ec0927d 100644
--- a/src/SentryIntegration.php
+++ b/src/SentryIntegration.php
@@ -18,7 +18,7 @@ class SentryIntegration {
    * @see https://www.php.net/manual/fr/errorfunc.constants.php
    * @var array<int>
    */
-  protected static array $php_error_types = array(
+  protected static $php_error_types = array(
     E_ERROR, E_PARSE, E_CORE_ERROR, E_COMPILE_ERROR, E_USER_ERROR,
     E_RECOVERABLE_ERROR,E_DEPRECATED,
   );
diff --git a/src/Session.php b/src/Session.php
index 010ddeb..11499d0 100644
--- a/src/Session.php
+++ b/src/Session.php
@@ -11,7 +11,7 @@ class Session {
    * Session max duration (in seconds, default: 12h)
    * @var int
    */
-  protected static int $max_duration = 12 * 60 * 60;
+  protected static $max_duration = 12 * 60 * 60;
 
   /**
    * Initialization
diff --git a/src/Tpl.php b/src/Tpl.php
index 30a13e1..9faf952 100644
--- a/src/Tpl.php
+++ b/src/Tpl.php
@@ -13,7 +13,7 @@ class Tpl {
    * Smarty object
    * @var \Smarty
    */
-  public static Smarty $smarty;
+  public static $smarty;
 
   /**
    * Smarty_Security object
@@ -25,25 +25,25 @@ class Tpl {
    * Core Smarty templates directory
    * @var string
    */
-  public static string $core_templates_directory;
+  public static $core_templates_directory;
 
   /**
    * Smarty templates directories path with their priority
    * @var array<string,int>
    */
-  public static array $templates_directories = array();
+  public static $templates_directories = array();
 
   /**
    * Smarty cache templates directory path
    * @var string
    */
-  public static string $templates_c_dir;
+  public static $templates_c_dir;
 
   /**
    * Enable/disable AJAX returned data debugging in logs
    * @var bool
    */
-  public static bool $_debug_ajax;
+  public static $_debug_ajax;
 
   /**
    * Core static directory
@@ -55,19 +55,19 @@ class Tpl {
    * Static directories path with their priority
    * @var array<string,array>
    */
-  private static array $static_directories = array();
+  private static $static_directories = array();
 
   /**
    * CSS files to load in next displayed page
    * @var array<string>
    */
-  private static array $css_files = array();
+  private static $css_files = array();
 
   /**
    * JavaScript files to load in next displayed page
    * @var array<string>
    */
-  private static array $js_files = array();
+  private static $js_files = array();
 
   /**
    * MIME type detector object
@@ -508,7 +508,7 @@ class Tpl {
    * @return bool
    */
   public static function initialized() {
-    return isset(self :: $smarty);
+    return self :: $smarty instanceof Smarty;
   }
 
   /**
diff --git a/src/Url.php b/src/Url.php
index 010f378..971ed25 100644
--- a/src/Url.php
+++ b/src/Url.php
@@ -48,7 +48,7 @@ class Url {
    * @see self :: initialization()
    * @var bool
    */
-  protected static bool $_api_mode;
+  protected static $_api_mode;
 
   /**
    * Initialization