Composer "config.platform" Mismatch - Resolving Against the Wrong PHP

config.platform.php in composer.json pins the PHP version Composer resolves against, regardless of the real interpreter. When that pin drifts from the PHP actually running in CI, you get installs that assume the wrong version - too new to run, or too old to use needed features.

What this error means

Composer resolves and installs cleanly, but either the lockfile targets a PHP older/newer than the runner, or runtime fails on a feature the faked platform claimed. php -v and config.platform.php disagree.

composer.json + CI log

// composer.json
"config": { "platform": { "php": "8.1.0" } }

$ php -v
PHP 8.3.4 (cli)
# Composer locks packages for 8.1, so an 8.3-only dependency is never selected,
# or a package capped at <8.2 is wrongly considered installable.

Common causes

The pinned platform version is stale

config.platform.php was set to match an old deploy target and never updated. The solver honours the pin, not the real PHP, so the resolved set targets the wrong version.

Platform pin set higher than the runtime

If the pin claims a newer PHP than the runner has, Composer may select packages that the actual interpreter cannot run, failing at runtime rather than install.

How to fix it

Align config.platform.php with the deploy target

Set the pin to the lowest PHP your production actually runs, and keep CI on that version (or test a matrix).

composer.json

{
  "config": {
    "platform": { "php": "8.3.0" }
  }
}

Inspect the effective platform

Ask Composer what platform it is resolving against versus the real interpreter.

Terminal

composer config platform.php
php -v
composer check-platform-reqs

Re-lock after changing the pin

A platform change affects resolution, so regenerate the lockfile.