Make a PHP app legible to an AI coding agent by giving it a concise map of the repository, reliable setup and verification commands, and clear guidance on conventions that the code alone does not explain. Keep detailed architecture and domain knowledge in maintained documentation, and point to it rather than turning one instruction file into a manual.
Start with a short, repository-specific map
Begin at the repository root with a guide that answers the questions an agent must resolve before changing code: What does the application do? Where are its main layers? How is it run and checked? Which local rules matter? Use real paths and commands from the project, not a generic PHP template.
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OpenAI describes a structured documentation knowledge base as the system of record for its agent-first engineering work, and cautions that an oversized AGENTS.md can crowd out the task and code context an agent also needs. Its concise principle: “give Codex a map, not a 1,000-page instruction manual.” OpenAI’s account of harness engineering is an example of that approach, not a universal file-size rule.
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- Purpose: a brief description of the app and its main users or workflows.
- Directory map: the real locations of application code, tests, configuration, migrations, and any important frontend or infrastructure code.
- Setup: prerequisites, the project’s actual installation and startup steps, and required services or environment variables. Describe variable names and how to obtain local values; do not put secrets in the guide.
- Checks: the commands contributors should run for tests, formatting, and static analysis, along with any required services or setup.
- Non-obvious rules: where new features belong, how migrations are handled, what is generated, and important dependency or error-handling conventions.
- Documentation pointers: links or paths to deeper architecture, domain, deployment, and troubleshooting material.
A useful instruction is actionable: “Put HTTP controllers in src/Controller; run composer test before submitting.” Include examples only when those paths and commands are true for your repository.
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Put each kind of guidance in the right place
Keep root instructions navigable. Put detailed explanations in documents that someone on the team can maintain, then point to them from the guide. For example, a domain workflow belongs in the domain documentation; deployment procedures belong in deployment documentation. Duplicating lengthy explanations across several instruction files makes them harder to keep consistent.
For Codex specifically, OpenAI documents AGENTS.md as a repository-instruction mechanism. Its prompting guide explains how global and project instruction files are discovered and combined according to scope. That behavior is product-specific: do not assume that every coding agent discovers the same filename or merges instructions the same way. Check the documentation for the exact agent and version you use, and add an agent-specific adapter when needed.
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OpenAI’s Codex prompting guide describes Codex’s instruction discovery. The Symfony AI repository’s guide offers a concrete PHP monorepo example of layout, commands, and upgrade guidance. Treat it as an example to learn from, not a required template for an application.
Use nested instructions selectively
Add directory-level guidance only when a subtree genuinely has different rules—for example, when a generated-code directory must not be edited directly or a package has its own test command. Check that nested guidance agrees with the root guide. Contradictions about where files belong or which command to run can send an agent in the wrong direction.
Make setup and checks reproducible
Document the commands a contributor can run in a clean, supported local environment. State prerequisites and dependencies explicitly: PHP version, required services, relevant environment configuration, and whether a command assumes dependencies have already been installed. Avoid copying credentials or machine-specific values into repository guidance.
Prefer the project’s existing scripts over a list of commands assembled from memory. If the repository provides a Composer script for tests or formatting, document that script and its expected result. Where multiple checks serve different purposes, distinguish them—for example, a fast test command from a broader suite—rather than leaving the agent to guess.
Rank #4
Keep command names, paths, and setup requirements synchronized with the working repository. Stale instructions are not harmless: an agent may follow them confidently, inspect the wrong layer, or report a check that it did not actually run.
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Agents have to infer intent from code, so accurate types make a practical difference. Add or correct property, parameter, and return types where they reflect actual behavior; keep annotations precise rather than using them to conceal uncertainty. PHPStan says that “Properly annotated and typehinted code (class properties, function and method arguments, return types) helps not only static analysis tools but also other people that work with the code to understand it.” PHPStan’s getting-started guide explains its setup and analysis workflow.
Run analysis on code your team owns and can fix. PHPStan specifically advises against analyzing third-party vendor code, whose errors are controlled by its maintainers. Use the project’s configured analyzer and level when one exists; address meaningful findings rather than treating a clean result from an arbitrarily narrow setup as proof that the whole application is sound.
PHPStan setup when the project does not already have it
PHPStan’s current getting-started page says the tool requires PHP 7.4 or newer to run and shows installation with Composer:
composer require --dev phpstan/phpstan
vendor/bin/phpstan analyse src testsIn that example, src and tests are the directories being analyzed; adjust them to match the project’s owned code. The executable is normally vendor/bin/phpstan after Composer installation. PHP and PHPStan requirements are version-sensitive, so check the documentation for the specific release you install and for the project’s supported PHP versions.
Validate whether an agent can use the map
Do not judge repository guidance by whether it sounds comprehensive. Test whether it helps an agent orient itself without inventing structure or commands. Give it a small, representative task and ask it first to summarize the application, identify the likely files to inspect, and name the checks it would run. Compare those claims against the repository and its working commands.
- Choose a bounded task that touches a real application area, such as tracing a request flow or locating the relevant test.
- Ask the agent to describe the app’s relevant layers and cite the repository paths it would inspect before making changes.
- Ask which setup assumptions and verification commands apply, including any needed services.
- Check each path, convention, and command yourself. Correct the guide or underlying documentation wherever the answer is wrong or unclear.
- Repeat after significant directory, tooling, or workflow changes so the map remains aligned with the code.
This is a practical orientation check, not a guarantee of agent correctness. The repository still needs normal review and testing for each change.
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