This code snippet lets you start an EC2 instance using the PHP SDK (and means you don't need to spin up a JVM for it)

ec2_start.php

<?php
// This code is released under a CC-Zero license
// https://creativecommons.org/publicdomain/zero/1.0/

// You need the AWS SDK - get the zip file from here: 
// http://pear.amazonwebservices.com/get/aws.zip
// And then unpack it to this directory.
require dirname(__FILE__) . '/aws-autoloader.php';

// Use the Ec2Client libraries by referring just to Ec2Client
use Aws\Ec2\Ec2Client;

// Technically, at this point, nothing has "failed", but
// nothing has succeeded
$failed = true;

// Try 5 times to start the instance
for ($try = 0; $failed && $try < 5; $try++) {
  try {
    // Create an Ec2Client object using the credentials below
    $client = Ec2Client::factory(array(
      // Credentials obtained by creating a user account in:
      // https://console.aws.amazon.com/iam/home?#users
      // Remember to add a full admin access user policy to the user
      'key'    => '',
      'secret' => '',
      'region' => 'us-east-1'
    ));

    // Now start the instance
    $result = $client->startInstances(array(
      // You must put an instance ID here
      'InstanceIds' => array('')
    ));
    $failed = false;
  } catch (Exception $e) {
    // If either the credentials are wrong, or EC2 has a problem
    // this will tell you what has happened.
    echo "Failed: " . $e->getMessage() . "\r\n";
    sleep(20);
  }
}

// If you've actually managed to start an instance
// this will tell you that it's started
if (!$failed) {
  $arrResult = $result['StartingInstances'][0];
  $was = $arrResult['PreviousState']['Name'];
  $is = $arrResult['CurrentState']['Name'];
  echo "Instance was: $was\r\nInstance  is: $is\r\n";
}