PHP Xdebug writes multiple error_log files for one request with fresh Laravel install

Question

I found an issue in my dev container that when I leave the Xdebug helper browser extension in debug mode, while my Xdebug server is actually not running, then multiple error log files are created.

I started to simplify the container, and it turned out that the issue is present even with a fresh Laravel install. I don't know whether this is a bug in PHP, Xdebug, Apache or Laravel.

My Dockerfile is:

FROM php:8.1-apache

RUN apt-get update && apt-get install -y curl unzip

RUN cd /tmp && curl -sS https://getcomposer.org/installer | php && \
    mv composer.phar /usr/local/bin/composer

RUN pecl install xdebug && docker-php-ext-enable xdebug

RUN echo "error_log = my_error_log" >> $PHP_INI_DIR/conf.d/php.ini && \
    echo "xdebug.mode = debug" >> $PHP_INI_DIR/conf.d/php.ini && \
    echo "xdebug.client_host = localhost" >> $PHP_INI_DIR/conf.d/php.ini

RUN sed -i 's#DocumentRoot .*#DocumentRoot /var/www/html/public#' /etc/apache2/sites-available/000-default.conf

ENV HOME /var/www
RUN chmod 777 $HOME

USER www-data

WORKDIR /var/www
RUN composer create-project laravel/laravel html

My docker-compose.yml file is:

version: "3"
services:
  example:
    container_name: example
    build: .

After I compose up this project, I made exactly one HTTP request:

$ docker compose exec example curl --head --cookie "XDEBUG_SESSION=random_stuff" localhost
HTTP/1.1 200 OK
Date: Sat, 28 Jan 2023 13:28:42 GMT
Server: Apache/2.4.54 (Debian)
X-Powered-By: PHP/8.1.14
Cache-Control: no-cache, private
Set-Cookie: XSRF-TOKEN=eyJpdiI6IlV0ZnBBYjh1VCtIaUdHMlF2TG52bXc9PSIsInZhbHVlIjoiei9TUUVCL29VeW82aHg5R1NNRm5LMmhKdEJlR2lYSDNic0M3d1F4K2gyeG05TThueG1zWHA0ai94b1E0UVpJeW5aemUxUTh5UEJtbkFDUnZ4SnhUZ0lkb2lxQnl0S0lGRDByQUtJSjdQOXA4YVhGTnNaK3ZDU3duZVBDbnFWNEwiLCJtYWMiOiJhOGViZTI2ODdlMzRiY2QzNDhkNzQwZDhiNWU0NTgzNmVlOTczNjBmNjJlMWQ1ODliOWZjYjZmY2U1OWY2M2Y3IiwidGFnIjoiIn0%3D; expires=Sat, 28 Jan 2023 15:28:42 GMT; Max-Age=7200; path=/; samesite=lax
Set-Cookie: laravel_session=eyJpdiI6IlJ5MVpaVVduZGdDbnZsMTA2QWpjNlE9PSIsInZhbHVlIjoibDhoMm1oa2U1MlgraFlFQjVyVWQ5Z0dSR0Z4QSsyckhXTjVvVUs4SGRlc3JtVDRTVUc2eXY1THkzbDgyMGJrL3UyN3Q2RFlkU2FQUVZxa1dEdThFZ3owVWZTUGlHWFlYd2NJR3dVQXNjMDhheUxVRXJPRFRPemY5UkszWFdCeFIiLCJtYWMiOiJjZTVjZjg0NDE4ODFkZTI3NWRlNWNjYTEzZTc5NGU5MjEyYmQ5YzdhZDUxZTNhNmNjMWE3OTM0NmQ1MDc4OGIwIiwidGFnIjoiIn0%3D; expires=Sat, 28 Jan 2023 15:28:42 GMT; Max-Age=7200; path=/; httponly; samesite=lax
Vary: Accept-Encoding
Content-Type: text/html; charset=UTF-8

but multiple error logs are created:

$ docker compose exec example cat /var/www/my_error_log
[28-Jan-2023 13:28:42 UTC] Xdebug: [Step Debug] Could not connect to debugging client. Tried: localhost:9003 (through xdebug.client_host/xdebug.client_port).

$ docker compose exec example cat /var/www/html/public/my_error_log
[28-Jan-2023 13:28:42 UTC] Xdebug: [Step Debug] Could not connect to debugging client. Tried: localhost:9003 (through xdebug.client_host/xdebug.client_port).

I know it may be a bad practice to use relative path in error_log directive. When I use absolute path, obviously only one file will be created but with twice the error message.

So why my PHP creates multiple error logs?

What I tried:

• use an older version of xdebug, starting from 3.0.0,
• use PHP 7.3, 8.0, 8.1 and 8.2 images (xdebug-3.1.6 is the latest version supports PHP 7.3),
• use the --no-dev option with Composer,
• use older version of Laravel, but the issue is present even in v6 branch.