ALTO \ Code Highlight

Server-side syntax highlighting for PHP applications, with semantic scopes, embedded languages, and no third-party PHP package dependencies at runtime.

Core highlighting runs entirely in PHP. It needs no browser runtime, Node.js process, or external service. Its parsers assign semantic scopes, so themes can distinguish a function definition from a call or a type definition from a reference.

Install

composer require alto/code-highlight

Requirements:

• PHP 8.4 or later;
• ext-mbstring;
• ext-tokenizer.

See the installation guide for verification and troubleshooting.

Quick start

<?php

use Alto\Code\Highlight\Highlighter;
use Alto\Code\Highlight\Theme\AltoTheme;

$theme = new AltoTheme();
$highlighter = new Highlighter($theme);
$code = '<?php echo "Hello, Alto!";';

echo '<style>'.$theme->getStylesheet().'</style>';
echo $highlighter->highlight($code, 'php');

highlight() returns escaped HTML inside <pre class="alto-highlight"><code>…</code></pre>. Emit a theme stylesheet once per page, then reuse the highlighter for every code block.

What it covers

• 27 languages: the PHP web stack plus common programming, markup, data, configuration, and query languages.
• Semantic highlighting: context-aware scopes for definitions, calls, types, variables, constants, and other language concepts.
• Embedded languages: CSS and JavaScript in HTML/SVG, fenced code in Markdown, and language blocks in Twig.
• Line controls: optional line numbers and selected-line emphasis.
• 12 built-in variants: seven theme families, including Alto, GitHub, Dracula, Polar, Cupertino, Noctis, and Solar.
• Theme compatibility: adapters for Highlight.js CSS, Prism CSS, and TextMate .tmTheme files.

Documentation

Guide	Contents
Documentation index	Choose the right guide
Getting started	Complete rendering, line numbers, and errors
Languages	Exact identifiers and language capabilities
Themes	Built-in variants and visual examples
Create a theme	Implement ThemeInterface
Embedded languages	HTML, SVG, Markdown, and Twig
Theme adapters	Highlight.js, Prism, and TextMate
Public API	Supported entry points and extension contracts
Examples	Compact examples and generated previews

The complete source examples are available in examples/languages/.

Languages

Use the lowercase identifier in the second argument to highlight():

bash        csharp       css          diff         dockerfile
dotenv      go           html         http         ini
java        javascript   json         makefile     markdown
php         python       ruby         rust         scss
sql         svg          swift        twig         typescript
xml         yaml

The special php-snippet identifier accepts PHP without an opening <?php tag. The language reference documents exact behavior and embedded-language support.

Line numbers and highlighted lines

$html = $highlighter->highlight(
    code: $code,
    language: 'php',
    lineNumbers: true,
    highlightLines: [2, 3],
);

Choose a theme

use Alto\Code\Highlight\Theme\GitHubTheme;

$light = new GitHubTheme(dark: false);
$dark = new GitHubTheme();

Browse the built-in theme matrix, learn how to create a theme, or reuse an existing stylesheet through a theme adapter.

Integrations

alto/twig-code-highlight adds blocks and filters for Twig applications. The core package remains framework-independent and can be used in Symfony controllers, Laravel views, static generators, or any PHP rendering pipeline.

Contributing

Issues and pull requests are welcome. Before proposing a change, run:

composer qa

Language parsers use fixtures under tests/Language/. Public showcase examples live separately under examples/languages/; they are short documentation samples rather than exhaustive parser tests.

License

ALTO Code Highlight is released under the MIT License.