Add some guidance on upgrading from v3 to v4

* Prevent deprecation notices for implode, json_encode and substr modifiers.
* unit tests

docs/upgrading.md

@@ -0,0 +1,38 @@
+# Upgrading from an older version
+
+## Upgrading from v3 to v4
+
+Smarty 4 is mostly identical to Smarty 3. Most notably, it adds support for PHP8 and drops support for PHP7.0 and below.
+Additionally, some deprecated features that have long been discouraged have been dropped from the language.
+
+### Muting PHP8 warnings
+If you simultaneously upgrade Smarty to v4 van PHP to v8, you may notice your error logs filling up with warnings about undefined or null template vars
+due to a change in how PHP handles these. This may be helpful to spot errors, but if you find this annoying, you can use
+`$smarty->muteUndefinedOrNullWarnings()` to make Smarty convert these warnings into notices.
+
+### ASP tags
+You can no longer user ASP-style tags like `<% %>` and `<%= %>` in your templates.
+Replace them with `{...}` tags.
+
+### SmartyBC
+Check your codebase for `SmartyBC`.
+We have dropped deprecated API calls that where only accessible through the SmartyBC class.
+
+### No more embedded PHP
+We have completely dropped support for `{php}` and `{include_php}` tags and embedded PHP in templates.
+Check your templates for this, and rewrite any embedded PHP blocks, by moving logic to your PHP files or by
+creating a [plugin function](./programmers/plugins/plugins-functions.md).
+
+### Other changes
+
+Search your code for the following changes:
+
+- `SMARTY_RESOURCE_CHAR_SET` and `SMARTY_RESOURCE_DATE_FORMAT` constants have been removed
+- `Smarty::muteExpectedErrors` and `Smarty::unmuteExpectedErrors` API methods have been removed
+- `Smarty::getVariable` method has been removed. Use [Smarty::getTemplateVars](programmers/api-functions/api-get-template-vars.md) instead.
+- [Smarty::registerResource](programmers/api-functions/api-register-resource.md) no longer accepts an array of callback functions
+
+
+
+
+

mkdocs.yml

@@ -26,7 +26,9 @@ markdown_extensions:
 
 nav:
   - Home: 'index.md'
-  - 'Getting started': 'getting-started.md'
+  - 'Getting started':
+    - Introduction: 'getting-started.md'
+    - 'Upgrading from an older version': 'upgrading.md'
   - 'Designers':
     - 'Basic Syntax':
         - Introduction: 'designers/language-basic-syntax/index.md'