When a major PHP version change (eg. 5.7 to 7.0, or 7.0 to 8.0) occurs, Amazon AWS Elastic Beanstalk typically does not allow for automated platform version updates but instead requires the cloning of an interface and a CNAME switch. For less available environments, use the following steps to upgrade-in-place using the AWS command line interface:

1. Take a look at the list of currently supported PHP platforms on Elastic Beanstalk; you’ll want to decide which Solution Stack Name (eg. “64bit Amazon Linux 2 v3.4.1 running PHP 8.1”) corresponds to the platform you wish to upgrade to.
2. In the Elastic Beanstalk web configuration page, get the environment ID for the environment you wish to upgrade (eg. “e-jwfjvy57r3”).
3. Run the aws command to upgrade the environment in place:
   aws elasticbeanstalk update-environment --solution-stack-name "64bit Amazon Linux 2 v3.4.1 running PHP 8.1" --environment-id "e-2iuwivbgzc" --region "ca-central-1"

Reference:

• Blue Keyboard – How to update PHP versions on Elastic Beanstalk environments