chore(tidy): clean up PHP example

[Shillaker]

Summary

• Pin version of aws-sdk-php (current versions break with dependency conflict)
• Make project ID a variable to avoid having it in a tfvars file
• Avoid copy-pasting in example using Terraform outputs

Checklist

• I have reviewed this myself.
• I have attached a README to my example. You can use this template as reference.
• I have updated the project README to link my example.

Details

More recent versions of the aws-sdk-php dependency break with an error of the form:

DEBUG - Fatal error: Declaration of RingCentral\Psr7\Response::getStatusCode() must be compatible with Psr\Http\Message\ResponseInterface::getStatusCode(): int in /home/app/vendor/ringcentral/psr7/src/Response.php on line 109 source=user stream=stdout