Use the PHP AWS SDK (http://docs.amazonwebservices.com/AWSSDKforPHP/latest/
See the RDS Section:
Something along these lines might work (extracted from example, slightly modified).
$rds = new AmazonRDS();
$response = $rds->create_db_instance('myInstance', 5, 'db.m1.small', 'MySQL', 'dbUser', 'dbPassword', array(
'DBName' => 'myDatabase',
'DBSecurityGroups' => 'default',
'PreferredMaintenanceWindow' => 'Sun:05:00-Sun:09:00',
'BackupRetentionPeriod' => 1,
'PreferredBackupWindow' => '03:00-05:00',
'MultiAZ' => true,
'AutoMinorVersionUpgrade' => true
));
// Success?
var_dump($response->isOK())
So that will create your new db, then simply follow the migration guide after downsizing your orginal DB at http://aws.amazon.com/articles/2933