retaining PHP variable after re-running the same script

I'm very new to PHP, and I'm trying to add features to an already existing app.

The following PHP code will be used each time there's a database request from the client side.

  <?php
   header("Access-Control-Allow-Origin: *");

   require_once("config/keychain.php");

   function decrypt($data, $key, $iv){
   $key = pack('H*', $key);
   $iv = pack('H*', $iv);
   return mcrypt_decrypt( MCRYPT_RIJNDAEL_128 , $key , $data ,  
   MCRYPT_MODE_CBC ,  $iv );
   }
   $request = (object)$_REQUEST;
   $dbConfig = (object)parse_ini_string(decrypt(file_get_contents("config/database.ini"), ENCRYPTION_KEY, ENCRYPTION_IV));
   require_once("include/UniversalDB.php");
   require_once("include/UniversalModel.php");
   require_once("include/UniversalController.php");
  //
  // Set DB Connection
  //
   $dbConnection = new UniversalDB();
   $dbConnection->init($dbConfig->host, $dbConfig->user, $dbConfig-
   >password, 2);
  //$universalDB->connect();
  require_once("boot.php");
  ?>

I'm trying to maintain one property from one of these requests values(the login $_REQUEST values).

I tried adding this to the previous script.

if(property_exists($request,'selectedDatabase')){
 $selectedDatabase = $request->selectedDatabase;

 }

I will get the $selectedDatabase initialized correctly. However, every time this script is running everything gets uninitialized.

I also tried making $selectedDatabase in $GLOBALS and making it static,but I lose the value when another request is coming.

Any ideas how can I maintain $selectedDatabase?

NOTE: The file where this script is written called "Index.php", and I'm not sure if it's the first script to load or not. However it seems like it!

Thank you,