YAML Parser Library

A PHP implementation of a YAML parser built for learning and understanding the YAML 1.2 specification.

Features

• Extensible Tag System: Define and register custom tags with ease
• Anchor and Alias Support: Automatic resolution of anchors and aliases
• Error Handling: Detailed error messages with line and column information
• Merge Key Support: Implements merge keys for mappings even though not defined in YAML 1.2

Installation

composer require maxbeckers/php-yaml-parser

Usage

Basic Parsing

use MaxBeckers\YamlParser\YamlParser;

$yamlParser = new YamlParser();
$data = $yamlParser->parseFile('config.yaml');

Custom Tag Handlers

// Register custom tag handler for environment variables
$yamlParser->getTagRegistry()->register(
    new CustomTagHandler('!env', function($value) {
        return getenv($value) ?: $value;
    })
);

// Use in YAML
// database_host: !env DATABASE_HOST

Architecture

The parser follows a multi-stage pipeline:

Input (string/file)
    ↓
Lexer (tokenization)
    ↓
Parser (AST building)
    ↓
Tag Processor (allows also custom tag handling)
    ↓
Resolver (anchors/aliases)
    ↓
Resolver (merge keys)
    ↓
Serializer (to PHP ArrayObject)

Background

I have written this YAML parser library in PHP to learn more about YAML. Already a lot of months ago someone said to me "YAML is a weird beast", that triggered my curiosity to learn more about it. So i started to read the YAML specification and implement a parser for it. While implementing the parser i found out that YAML is indeed a weird beast. There are many edge cases and special cases that make it hard to implement a fully compliant parser. Also the specification itself is not always clear and sometimes even contradictory. So i had to make some decisions on how to handle certain cases. I tried to follow the specification as closely as possible, but there are some cases where i had to deviate from it. For example, the specification does not define how to handle merge keys in YAML 1.2, but i decided to implement it anyway, because it is a useful feature. This means that this library is not fully compliant with the YAML specification and probably never will be 100% compliant. Use at your own risk. But i make it publicly available in the hope that it might be useful to someone.

Todos

• Performance optimizations for large YAML files
• Code cleanup and refactoring
• Additional unit tests for edge cases
• Implement parsing for currently skipped test cases
• Optimize handling for special cases
• Add configurable options for parsing behavior
• Improve metadata handling for mappings and sequences
• Make all tests from the YAML test suite pass (currently 124/402 are skipped)

Contributing

Contributions are welcome! Please feel free to submit issues or pull requests.

Acknowledgments

Built with reference to the YAML 1.2.2 Specification.