base.html.twig

{% stylesheets '%kernel.root_dir%/Resources/less/site/frontend.less' filter='lessphp' output='css/style.css' %}
<link rel="stylesheet" href="{{ asset_url }}" />
{% endstylesheets %}

composer.json

// Add it to your composer.json
        "leafo/lessphp":                    "dev-master"

config.yml

# Example of configuration for symfony/assetic/lessphp
assetic:
    debug:          "%kernel.debug%"
    use_controller: false
    bundles:        [ FOSUserBundle ]
    filters:
        cssrewrite: ~
        lessphp:
            file: %kernel.root_dir%/../vendor/leafo/lessphp/lessc.inc.php
            apply_to: "\.less$" # Apply the filter only on .less file, keep simple css files safe (but less/css are compatible)