Recently, I have gotten involved in a project where, among other things, we needed to create EC2 instances from a web application.
We are using CloudFormation templates to create VPC with public and private subnets, Internet gateway, Security Groups, NACLs, etc.
We have decided to have separate stacks for network infrastructure, servers, ELB and auto-scalers, and we are also considering a separate security stack.
What makes this possible is using the cross-stack reference option.
“To create a cross-stack reference, use the Export output field to flag the value of a resource output for export. Then, use the Fn::ImportValue intrinsic function to import the value”
So simply put, you create a new stack where you import values from a parent stack where those same values are exported in the “Outputs” section.
In this example, I am exporting values from my main template to be consumed later by my EC2 instances template:

"Outputs" : {
"PrivateSubnet" : {
"Description" : "The subnet ID to use for web servers behind an ELB",
"Value" : { "Ref" : "PrivateWebServer1" },
"Export" : { "Name" : {"Fn::Sub": "${AWS::StackName}-SubnetID" }}
},
"BasicSecurityGroup" : {
"Description" : "The security group ID to use for private web servers",
"Value" : { "Fn::GetAtt" : ["BasicSecurityGroup", "GroupId"] },
"Export" : { "Name" : {"Fn::Sub": "${AWS::StackName}-SecurityGroupID" }}
}
}

Notice that I am first referencing the value that need to be exported using the “Ref” function.
Next, I am using the Export parameter to export the SubnetID, and in the next declaration the SecurityGroupID of a Stack Name.
Using the Fn-Sub function, I am substituting the $(AWS::StackName) variable with an input parameter.
This Parameter is defined in our Parameters section of the child template:

“"Parameters": {
"NetworkStackName": {
"Description": "Name of an active CloudFormation stack that contains the networking resources, such as the subnet and security group, that will be used in this stack.",
"Type": "String",
"MinLength" : 1,
"MaxLength" : 255,
"AllowedPattern" : "^[a-zA-Z][-a-zA-Z0-9]*$",
"Default" : "Your-Parent-Template"
}
}

This is the complete child stack that will import values from its parent stack:

{
"AWSTemplateFormatVersion": "2010-09-09",

"Description" : "CF stack to create an EC2 instance, with Parent VPC name as parameter",

"Parameters": {
"NetworkStackName": {
"Description": "Name of an active CloudFormation stack that contains the networking resources, such as the subnet and security group, that will be used in this stack.",
"Type": "String",
"MinLength" : 1,
"MaxLength" : 255,
"AllowedPattern" : "^[a-zA-Z][-a-zA-Z0-9]*$",
"Default" : "MyVPC3"
}
},

"Resources": {

"WebServer": {

"Type": "AWS::EC2::Instance",

"Properties": {

"InstanceType" : "t2.micro",

"ImageId" : "xxxxxxxx",

"KeyName" : "xxxxxxxxx",

"NetworkInterfaces": [ {
"AssociatePublicIpAddress": "false",
"DeviceIndex": "0",
"GroupSet" : [{ "Fn::ImportValue" : {"Fn::Sub": "${NetworkStackName}-SecurityGroupID" } }],

"SubnetId":{ "Fn::ImportValue" : {"Fn::Sub": "${NetworkStackName}-SubnetID" } }
} ]

}
}
}
}

when the child stack is ran, it will create an EC2 machine inside the VPC and subnet created in the parent stack, and assign it the appropriate security group.
After testing that everything works, now it’s time to create an EC2 instance from a web application.
We will use PHP, and the well documented AWS SDK for PHP.
Best way to install the SDK is to use Composer.
Go through the trouble of installing it because it’s worth it. You won’t have to worry about troubleshooting issues with dependencies, because composer takes care of that.
I am using PHP with IIS on a windows server.
Download the exe file, and install composer. Check if it’s installed:

Next, install the PHP SDK by following these instructions, and run the following command from the vendor directory of your PHP project: (If PHP binary is not in your path, then specify the full command path)

php -r "eval('?>'.file_get_contents('https://getcomposer.org/installer'));"

To use the PHP SDK for any AWS service, you will need to first start with creating a client. For more information on how to get started with calling an AWS service API, read this guide:
“The basic usage pattern of the SDK is that you instantiate a Client object for the AWS service you want to interact with“

My Cloudformation service client looks like this: (You can use credentials online, or use a profile in your credentials file. For security, use profiles!)

<?php require 'C:\awscf\vendor\autoload.php'; //Create a client use Aws\CloudFormation\CloudFormationClient; $client = CloudFormationClient::factory(array( 'region' => 'us-east-1',
    'credentials' => [
        'key'    => 'xxxxxxxxxxxxxxxxxxx',
        'secret' => 'xxxxxxxxxxxxxxxxxxxx',
    ],
));

// Create your stack stored on S3 that imports data from your network stack
    $result = $client->createStack(array(
    // StackName is required
    'StackName' => 'Webserver1',
    'TemplateURL' => 'https://s3.amazonaws.com/yourbucket/yourfile.template',
    ));
?>

Very straight forward way of creating a stack from your PHP web application. We have only used 2 elements of the array to keep it simple, but you are welcome to take advantage of the full capabilities of the API call.
For example, you could pass on your parameter of parent stack name to the child CF stack, instead of using the default.

I hope you found this article helpful to get you started quickly with using the AWS cloudformation PHP SDK.