smarty/docs/api/extending/extensions.md

# Creating an extension

In order to organize your custom tags and modifiers, you can create an Extension.
In fact, most of Smarty itself is organized into two extensions:

- the core extension, which provides the basic language tags such as `{if}`, `{for}` and `{assign}`. 
- the default extension, which provides all default modifiers such as `|escape`,  `|nl2br` and  `|number_format` 
  and tags such as `{html_image}`, `{mailto}` and `{textformat}` that are enabled by default, but not necessarily universal.

> ** Note **
> 
> There is also the 'BCPluginsAdapter' extension, which does not add any new functionality, but
> wraps calls to deprecated methods such as `Smarty\Smarty::addPluginsDir()` and `Smarty\Smarty::loadFilter()`.

In order to write your own custom extension, you must write a class that implements `Smarty\Extension\ExtensionInterface`.
However, it is usually easier to extend `Smarty\Extension\Base` which provides empty implementation for each of the methods
required by `Smarty\Extension\ExtensionInterface`. This allows you to only override the method(s) you need.

Example:
```php
<?php

use Smarty\Extension\Base;

class MyExtension extends Base {

    public function getModifierCompiler(string $modifier): ?\Smarty\Compile\Modifier\ModifierCompilerInterface {

		switch ($modifier) {
			case 'array_escape': return new MyArrayEscapeModifierCompiler();
			case 'array_unescape': return new MyArrayUnescapeModifierCompiler();
        }

		return null;
	}
}

```
Another example, that would allow you to use any valid PHP callable as a modifier in your templates:

```php
<?php

use Smarty\Extension\Base;

class MyCallablePassThroughExtension extends Base {

    public function getModifierCallback(string $modifierName) {
    
        if (is_callable($modifierName)) {
            return $modifierName;
        }
    
		return null;
	}
}

```

Writing an extension allows you to add a group of tags, block tags and modifiers to the Smarty language.
It also allows you to register pre-, post- and output-filters in a structured way. 
The files in `src/Extension/` in the `smarty/smarty` dir should give you all the information you need to start
writing your own extension.