How to stop error_log() from prefixing my json logs with “NOTICE: PHP message:”

Using php-fpm and nginx in a docker container. My custom php error handler builds a comprehensive JSON data and outputs it using error_log(). But somehow it is always prefixed with NOTICE: PHP message:, which ruins the JSON format reader in Grafana – because now the output is not a valid json string.

How do I stop it from prefixing my log strings?