smarty/docs/designers/language-custom-functions/language-function-eval.md

# {eval}

`{eval}` is used to evaluate a variable as a template. This can be used
for things like embedding template tags/variables into variables or
tags/variables into config file variables.

## Attributes

| Attribute Name | Required | Description                                          |
|----------------|----------|------------------------------------------------------|
| var            | Yes      | Variable (or string) to evaluate                     |
| assign         | No       | The template variable the output will be assigned to |

If you supply the `assign` attribute, the output of the `{eval}`
function will be assigned to this template variable instead of being
output to the template.

> **Note**
>
> -   Evaluated variables are treated the same as templates. They follow
>     the same escapement and security features just as if they were
>     templates.
>
> -   Evaluated variables are compiled on every invocation, the compiled
>     versions are not saved! However, if you have [caching](../../programmers/caching.md)
>     enabled, the output will be cached with the rest of the template.
>
> -   If the content to evaluate doesn't change often, or is used
>     repeatedly, consider using
>     `{include file="string:{$template_code}"}` instead. This may cache
>     the compiled state and thus doesn't have to run the (comparably
>     slow) compiler on every invocation.

## Examples

The contents of the config file, `setup.conf`.

```ini
emphstart = <strong>
emphend = </strong>
title = Welcome to {$company}'s home page!
ErrorCity = You must supply a {#emphstart#}city{#emphend#}.
ErrorState = You must supply a {#emphstart#}state{#emphend#}.
```

Where the template is:

```smarty
{config_load file='setup.conf'}

{eval var=$foo}
{eval var=#title#}
{eval var=#ErrorCity#}
{eval var=#ErrorState# assign='state_error'}
{$state_error}
```
      
The above template will output:

```html
This is the contents of foo.
Welcome to Foobar Pub & Grill's home page!
You must supply a <strong>city</strong>.
You must supply a <strong>state</strong>.
```

This outputs the server name (in uppercase) and IP. The assigned
variable `$str` could be from a database query.

```php
<?php
    $str = 'The server name is {$smarty.server.SERVER_NAME|upper} '
           .'at {$smarty.server.SERVER_ADDR}';
    $smarty->assign('foo',$str);
```

Where the template is:

```smarty
{eval var=$foo}
```