Smarty : substr a variable

Use the PHP function:

{$var|substr:0:30}

or mb_substr for UTF-8 variables:

{$var|mb_substr:0:30}

Quoting from the Variable Modifiers section of the documentation:

All php-functions can be used as modifiers implicitly, as demonstrated in the example above. However, using php-functions as modifiers has two little pitfalls:

• First - sometimes the order of the function-parameters is not the desirable one. Formatting $foo with {"%2.f"|sprintf:$foo} actually works, but asks for the more intuitive, like {$foo|string_format:"%2.f"} that is provided by the Smarty distribution.
• Secondly - if $security is enabled, all php-functions that are to be used as modifiers have to be declared trusted in the MODIFIER_FUNCS element of the $security_settings array.

[bold added]

This is actually difficult to find in the documentation by searching, which (as of this writing) turns up zero results for "php" or "substr".

You should use truncate modifier:

{$var|truncate:30}

More information here.