Options

The tag and filter accept the same options:

Option	Type	Default	Effect
`line_numbers`	`bool`	`false`	Adds a numbered span to every source line.
`highlight_lines`	`array<int>`	`[]`	Adds `alto-highlighted` to the selected line numbers.

Set defaults when creating the extension:

<?php

use Alto\Twig\CodeHighlight\CodeHighlightExtension;

$extension = new CodeHighlightExtension(defaultOptions: [
    'line_numbers' => true,
    'highlight_lines' => [2],
]);

Options passed by a tag or filter replace matching defaults for that call. The highlight_lines array is replaced rather than combined. Only positive integers are retained; a non-array value becomes an empty list. Enable line_numbers to make the selected line styling visible.

Themes

Pass a configured Code Highlight instance to the extension, then use that same instance when registering the runtime:

<?php

use Alto\Code\Highlight\Highlighter;
use Alto\Code\Highlight\Theme\AltoTheme;
use Alto\Twig\CodeHighlight\CodeHighlightExtension;

$highlighter = new Highlighter(new AltoTheme());
$extension = new CodeHighlightExtension($highlighter);

The default is AltoTheme. See Code Highlight's themes for built-in themes, adapters, custom themes, and stylesheet output. Emit the chosen theme's stylesheet once per page.

Line-number appearance is controlled by application CSS. Target .alto-line-number and .alto-line-number.alto-highlighted when customizing it.