How to use this image

All examples in this guide use the public image. If you’ve mirrored the repository for your own use (for example, to your Docker Hub namespace), update your commands to reference the mirrored image instead of the public one.

For example:

• Public image: dhi.io/<repository>:<tag>
• Mirrored image: <your-namespace>/dhi-<repository>:<tag>

For the examples, you must first use docker login dhi.io to authenticate to the registry to pull the images.

What’s included in this PHP image

This Docker Hardened PHP image includes PHP runtime and essential tools in a single, security-hardened package:

• php: PHP command-line interpreter and runtime environment (non-FPM variants only)
• php-fpm: FastCGI Process Manager for serving PHP applications (FPM variant only)
• phpdbg: a lightweight, powerful, easy to use debugging platform for PHP (dev variant only)
• Essential PHP extensions built-in/statically compiled
• Full phpize environment (phpize, php-config, compiler, PHP source code) for building extensions (dev variant only)

Start a PHP image

Run the following command and replace <tag> with the image variant you want to run.

docker run dhi.io/php:<tag> --version

Common PHP use cases

Running a PHP script

Execute PHP scripts directly from the command line.

docker run --rm -v $(pwd):/app -w /app dhi.io/php:<tag> php script.php

Serving a PHP application with PHP-FPM

Use the FPM variant to serve PHP applications through a FastCGI gateway like Nginx.

FROM dhi.io/php:<tag>-fpm
COPY . /var/www/html
EXPOSE 9000

Customizing PHP configuration with $PHP_INI_DIR

Configure PHP settings by adding custom ini files to the PHP configuration directory.

FROM dhi.io/php:<tag>
COPY custom-php.ini $PHP_INI_DIR/conf.d/

Building custom PHP extensions (dev variant)

Use the dev variant to compile and install additional PHP extensions.

Multi-stage build:

FROM dhi.io/php:<tag>-dev AS builder
WORKDIR /tmp

Example of building Redis extension manually:

RUN pecl install redis

FROM dhi.io/php:<tag>-fpm
COPY --from=builder $PHP_PREFIX/lib/php/extensions $PHP_PREFIX/lib/php/extensions

Add extension configuration:

RUN echo "extension=redis.so" > $PHP_INI_DIR/conf.d/redis.ini

Non-hardened images vs. Docker Hardened Images

Key differences

Why no shell or package manager?

Docker Hardened Images prioritize security through minimalism:

• Reduced attack surface: Fewer binaries mean fewer potential vulnerabilities
• Immutable infrastructure: Runtime containers shouldn't be modified after deployment
• Compliance ready: Meets strict security requirements for regulated environments

The hardened images intended for runtime don't contain a shell nor any tools for debugging. Common debugging methods for applications built with Docker Hardened Images include:

• Docker Debug⁠ to attach to containers
• Docker's Image Mount feature to mount debugging tools
• Ecosystem-specific debugging approaches

Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

For example, you can use Docker Debug:

docker debug <image-name>

or mount debugging tools with the Image Mount feature:

docker run --rm -it --pid container:my-php \
  --mount=type=image,source=dhi.io/busybox,destination=/dbg,ro \
  dhi.io/php:<tag> /dbg/bin/sh

Image variants

Docker Hardened Images come in different variants depending on their intended use.

Runtime variants are designed to run your application in production. These images are intended to be used either directly or as the FROM image in the final stage of a multi-stage build. These images typically:

• Run as the nonroot user
• Do not include a shell or a package manager
• Contain only the minimal set of libraries needed to run the app

Build-time variants typically include dev in the variant name and are intended for use in the first stage of a multi-stage Dockerfile. These images typically:

• Run as the root user
• Include a shell and package manager
• Are used to build or compile applications

PHP-specific variants

• fpm: FastCGI Process Manager variant for serving PHP applications through a FastCGI gateway (e.g., Nginx, Apache). This is the recommended way to serve PHP applications in production.
• dev: Development variant that includes build tools, a shell, APT package manager, PECL for extension management, and full compiler/phpize environment as well as the PHP source code for building custom extensions.

Migrate to a Docker Hardened Image

To migrate your application to a Docker Hardened Image, you must update your Dockerfile. At minimum, you must update the base image in your existing Dockerfile to a Docker Hardened Image. This and a few other common changes are listed in the following table of migration notes:

The following steps outline the general migration process.

1. Find hardened images for your app.

   A hardened image may have several variants. Inspect the image tags and find the image variant that meets your needs. For PHP applications, you'll typically want the -fpm variant for production.

2. Update the base image in your Dockerfile.

   Update the base image in your application's Dockerfile to the hardened image you found in the previous step. For PHP applications, this is typically going to be an image tagged as -dev because it has the tools needed to install packages and dependencies.

3. For multi-stage Dockerfiles, update the runtime image in your Dockerfile.

   To ensure that your final image is as minimal as possible, you should use a multi-stage build. All stages in your Dockerfile should use a hardened image. While intermediary stages will typically use images tagged as -dev, your final runtime stage should use a non-dev image variant like -fpm.

4. Install additional packages

   Docker Hardened Images contain minimal packages in order to reduce the potential attack surface. You may need to install additional packages in your Dockerfile. Inspect the image variants to identify which packages are already installed.

   Only images tagged as -dev typically have package managers. You should use a multi-stage Dockerfile to install the packages. Install the packages in the build stage that uses a -dev image. Then, if needed, copy any necessary artifacts to the runtime stage that uses a non-dev image.

   For Alpine-based images, you can use apk to install packages. For Debian-based images, you can use apt-get to install packages.

Troubleshoot migration

General debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. The recommended method for debugging applications built with Docker Hardened Images is to use Docker Debug⁠ to attach to these containers. Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

Migrating from Apache mod_php

The Docker Hardened PHP image does not support the Apache mod_php variant. Applications using the -apache2 variant of the Docker Official Image must migrate to use PHP-FPM with a separate web server.

Example migration approach using Docker Compose:

version: "3.8"
services:
  php:
    image: dhi.io/php:<tag>-fpm
    volumes:
      - ./src:/var/www/html

  nginx:
    image: nginx:alpine
    ports:
      - "80:80"
    volumes:
      - ./src:/var/www/html
      - ./nginx.conf:/etc/nginx/conf.d/default.conf
    depends_on:
      - php

Example nginx configuration for PHP-FPM:

server {
    listen 80;
    server_name localhost;
    root /var/www/html;
    index index.php index.html;

    location / {
        try_files $uri $uri/ /index.php?$query_string;
    }

    location ~ \.php$ {
        fastcgi_pass php:9000;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
    }

    location ~ /\.ht {
        deny all;
    }
}

Building custom PHP extensions

Extensions not included in the base image must be built using the -dev variant. While Docker Hardened Images don't include the docker-php-ext-* helper scripts found in Docker Official Images, the PHP source code is available in $PHP_SRC_DIR and a full phpize environment is present in the -dev variant.

Note that the mhash compatibility layer (deprecated since PHP 7.0.0) is not enabled - applications should use the modern hash extension instead.

Use a multi-stage build to compile extensions using the standard PHP build process.

Building an extension from PECL:

FROM dhi.io/php:<tag>-dev AS builder
WORKDIR /tmp
RUN pecl download redis \
    && tar xzf redis-*.tgz \
    && cd redis-* \
    && phpize \
    && ./configure \
    && make \
    && make install

FROM dhi.io/php:<tag>-fpm
COPY --from=builder /usr/local/lib/php/extensions /usr/local/lib/php/extensions
RUN echo "extension=redis.so" > $PHP_INI_DIR/conf.d/redis.ini

Building an extension from source:

FROM dhi.io/php:<tag>-dev AS builder
WORKDIR /tmp
RUN git clone https://github.com/phpredis/phpredis.git \
    && cd phpredis \
    && phpize \
    && ./configure \
    && make \
    && make install

FROM dhi.io/php:<tag>-fpm
COPY --from=builder /usr/local/lib/php/extensions /usr/local/lib/php/extensions
RUN echo "extension=redis.so" > $PHP_INI_DIR/conf.d/redis.ini

Legacy mhash functions

The mhash extension has been deprecated since PHP 7.0.0 and its functionality has been integrated into the hash extension. Docker Hardened PHP images do not include the mhash compatibility layer. If your application uses legacy mhash_* functions, you should update your code to use the modern hash equivalents:

• Replace mhash() with hash()
• Replace mhash_get_hash_name() with hash_algos()
• Replace mhash_get_block_size() with a custom implementation using hash()
• Replace mhash_count() with count(hash_algos())

Permissions

By default image variants intended for runtime, run as the nonroot user. Ensure that necessary files and directories are accessible to the nonroot user. You may need to copy files to different directories or change permissions so your application running as the nonroot user can access them.

Privileged ports

Non-dev hardened images run as a nonroot user by default. As a result, applications in these images can't bind to privileged ports (below 1024) when running in Kubernetes or in Docker Engine versions older than 20.10. PHP-FPM's default port 9000 works without issues.

No shell

By default, image variants intended for runtime don't contain a shell. Use dev images in build stages to run shell commands and then copy any necessary artifacts into the runtime stage. In addition, use Docker Debug to debug containers with no shell.

Entry point

Docker Hardened Images may have different entry points than images such as Docker Official Images. Use docker inspect to inspect entry points for Docker Hardened Images and update your Dockerfile if necessary.