1. Create a new Policy name 'SellingPartnerAPI_Policy'
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "execute-api:Invoke",
            "Resource": "arn:aws:execute-api:*:*:*"
        }
    ]
}

2. Create a new role name 'SellingPartner_Role'
3. Attach the above policy to the role you created
4. Create a new user 'SellingPartner_User'
5. Attached the SellingPartnerAPI_Policy to the current user
6. Create a new Inline Policy `STS_SellingPartnerRole`
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "sts:AssumeRole",
            "Resource": "arn:aws:iam::961637079591:role/SellingPartnerRole"
        }
    ]
}

PHP Test Script

amazon_sapi.zip