Support for configuration schema metadata

[ralflang]

Context

Related to horde/hordectl#2 - hordectl needs to provide interactive configuration with hints about required/optional fields.

Currently, configuration validation and schema information lives in:

1. Horde application XML config files (horde/base/config/*.xml)
2. Duplicated in web UI forms
3. Potentially duplicated in CLI (hordectl)

Problem

PhpConfigFile is excellent for reading/writing PHP config files while preserving structure, but it doesn't provide:

• Schema/metadata about configuration structure
• Validation rules
• Required vs optional field information
• Type information
• Help text/descriptions

This leads to duplication of validation logic across web UI and CLI tools.

Proposed Enhancement

Add optional schema support to PhpConfigFile:

Option 1: Schema as separate object

$schema = new ConfigSchema();
$schema->defineField('sql.phptype', [
    'type' => 'string',
    'required' => true,
    'enum' => ['mysql', 'pgsql', 'sqlite'],
    'help' => 'Database type'
]);

$file = new PhpConfigFile('config.php', schema: $schema);
$file->validate(); // Check against schema

Option 2: Schema from external source

$schema = ConfigSchema::fromXml('horde/config/conf.xml');
$file = new PhpConfigFile('config.php', schema: $schema);

Option 3: Schema embedded in config file

Use PHP 8 attributes or docblocks in generated config:

// Auto-generated from schema
class HordeConfig {
    #[Required, Type('string'), Enum(['mysql', 'pgsql'])]
    public string $sql_phptype;
}

Use Cases

1. CLI validation - hordectl can validate config before saving
2. Interactive prompts - Show required vs optional fields
3. Documentation - Generate config docs from schema
4. Migration - Validate config after upgrades
5. Type safety - Catch config errors early

Long-term Vision

See horde/hordectl#2 for broader architectural discussion:

• Migrate from XML-based schema to PHP class definitions
• Use PHP attributes for metadata
• Single source of truth for validation
• XML as output format for backwards compatibility

Questions

1. Should PhpConfigFile handle validation, or just provide schema access?
2. Should this be a separate package (horde/config-schema)?
3. How to handle backwards compatibility with existing XML schemas?
4. Performance implications of schema validation?

Related Issues

• Interactive mode should provide hints about missing required attributes per driver hordectl#2 - Interactive mode hints
• Future horde/core issue - Schema migration plan