InMemoryDB php class psuedo code

database.php

<?php

class Database
{
  /**
   * Storage
   
   * @var array
   */
  protected $database = array();

  /**
   * Transactions
   
   * @var array
   */
  protected $transactions = array();

  /**
   * Value usage counter
   
   * @var array
   */
  protected $valueUsage = array();

  /**
   * Set value
   
   * @param $name
   * @param bool $value
   */
  protected function setValue($name,$value=false)
  {
    if(isset($this->database[$name])) {
      $this->valueUsage[$this->database[$name]] -= 1;
      unset($this->database[$name]);
    }

    if ($value) {
      $this->database[$name] = $value;
      // IDEA: if values are mostly large, we can keep their hashes as $valueUsage key
      $this->valueUsage[$value] = isset($this->valueUsage[$value]) ? $this->valueUsage[$value] + 1 : 1;
    }
  }

  /**
   * Keep records about value change
   
   * @param $name
   */
  protected function addTransactionChange($name)
  {
    if ($this->transactions) {
      $lastTransaction = end($this->transactions);
      if (!isset($lastTransaction[$name])) {
        $this->transactions[key($this->transactions)][$name] = $this->get($name);
      }
    }
  }

  /**
   * Open a transactional block
   */
  public function begin()
  {
    array_push($this->transactions, array());
  }

  /**
   * Rollback all of the commands from the most recent transaction block
   *
   * @throws Exception
   */
  public function rollback()
  {
    if ($this->transactions) {
      $lastTransaction = end($this->transactions);
      foreach($lastTransaction as $name => $value) {
        $this->setValue($name,$value);
      }
      unset($this->transactions[key($this->transactions)]);
    } else {
      throw new Exception("INVALID ROLLBACK");
    }
  }

  /**
   * Permanently store all of the operations from any presently open transactional blocks
   */
  public function commit()
  {
    $this->transactions = array();
  }

  /**
   * Return the value stored under the variable $name
   
   * @param $name
   * @return string
   */
  public function get($name)
  {
    return isset($this->database[$name]) ? $this->database[$name] : 'null';
  }

  /**
   * Set variable
   
   * @param $name
   * @param $value
   */
  public function set($name,$value)
  {
    $this->addTransactionChange($name);
    $this->setValue($name,$value);
  }

  /**
   * Unset variable
   
   * @param $name
   */
  public function delete($name)
  {
    $this->addTransactionChange($name);
    $this->setValue($name);
  }

  /**
   * Returns the number of variables equal to $value
   
   * @param $value
   * @return int
   */
  public function numEqualTo($value)
  {
    return isset($this->valueUsage[$value]) ? $this->valueUsage[$value] : 0;
  }
}

index.php

<?php

require_once('database.php');

$database = new Database();
if(!defined('STDIN'))  define('STDIN',  fopen('php://stdin',  'r'));
if(!defined('STDOUT')) define('STDOUT', fopen('php://stdout', 'w'));
if(!defined('STDERR')) define('STDERR', fopen('php://stderr', 'w'));
fwrite(STDOUT, "\nHey! Program will terminate on command 'END'\n\n");

$data = array();

do {
  $command = trim(fgets(STDIN));
  $data[] = explode(' ', $command);
} while ($command != 'END');

fwrite(STDOUT, "-- OUTPUT --\n");

try {
  foreach ($data as $cmd) {
    switch ($cmd[0]) {
      case 'BEGIN':
        $database->begin();
        break;
      case 'ROLLBACK':
        $database->rollback();
        break;
      case 'COMMIT':
        $database->commit();
        break;
      case 'GET':
        echo $database->get($cmd[1])."\n";
        break;
      case 'SET':
        $database->set($cmd[1],@$cmd[2]);
        break;
      case 'UNSET':
        $database->delete($cmd[1]);
        break;
      case 'NUMEQUALTO':
        echo $database->numEqualTo($cmd[1])."\n";
        break;
      case 'END':
        exit(0);
        break;
      default:
        fwrite(STDOUT, "There is no command $cmd[0]!\n");
        exit(0);
        break;
    }
  }
} catch(Exception $e) {
  fwrite(STDOUT, $e->getMessage());
}

This is just a pseudo code. Not intended for production. Please note that this code is checked for its correctness. A lot of improvements may be needed.