interface DatabaseInterface{

/**
 * @return bool
 */
function connect();

/**
 * @return void
 */
function disconnect();

/**
 * @param string $tableName
 * @param array $columns
 * @param array $values
 * @return mixed
 */
function insert($tableName, $columns, $values);

/**
 * @param string $tableName
 * @param array $conditions
 * @param array $columns
 * @param array $values
 * @return mixed
 */
function update($tableName, $columns, $values, $conditions);

/**
 * @param string $tableName
 * @param string $columns
 * @param array $conditions
 * @param int $limit
 * @param int $offset
 * @return mixed
 */
function select($tableName, $columns,  $conditions, $limit, $offset);

/**
 * @param string $tableName
 * @param array $conditions
 * @return mixed
 */
function delete($tableName, $conditions);

/**
 * @param string $tableName
 * @return array
 */
function fetchFields($tableName);
}

interface MapperInterface{

function findById($id);

function save();

function loadClassProperties();
}

class MysqlAdapter implements DatabaseInterface
{
private $host = '';

private $username = '';

private $password = '';

private $dbName = '';

private $port = '';

private $socket = '';

private $_mysqli;

function __construct($host, $username, $password, $dbName, $port = null, $socket = null)
{
    $this->host = $host;
    $this->username = $username;
    $this->password = $password;
    $this->dbName = $dbName;
    $this->port = $port;
    $this->socket = $socket;
}

function connect()
{
    $this->_mysqli = new mysqli($this->host, $this->username, $this->password, $this->dbName, $this->socket);
    if ($this->_mysqli->connect_error) {
        return false;
    }
    return true;
}

function disconnect()
{
    if (isset($this->_mysqli)) {
        $this->_mysqli->close();
    }
}

/**
 * @param string $tableName
 * @param array $columns
 * @param array $values
 * @return mixed
 */
function insert($tableName, $columns, $values)
{
    $query = "INSERT INTO $tableName $columns VALUES $values";
    return $this->_mysqli->query($query);
}

/**
 * @param string $tableName
 * @param array $conditions structure -> column => (operator, value, logical_operator) e.g id => (>, 5, AND)
 * @param array $columns
 * @param array $values
 * @return mixed
 */
function update($tableName, $columns, $values, $conditions)
{
    $updateString = $this->generateUpdateString($columns, $values);
    $whereString = $this->generateWhereString($conditions);
    $query = "UPDATE $tableName SET  $updateString WHERE  $whereString";
    $result = $this->_mysqli->query($query);
    return $result;
}

/**
 * @param string $tableName
 * @param string $columns
 * @param array $conditions
 * @param int $limit
 * @param int $offset
 * @return mixed
 */
function select($tableName, $columns, $conditions, $limit = null, $offset = null)
{
    $query = "SELECT $columns FROM $tableName";
    if (!empty($conditions)) {
        $whereString = $this->generateWhereString($conditions);
        $query .= " WHERE $whereString";
    }
    if (isset($limit) && isset($offset)) {
        $query .= "LIMIT $limit OFFSET $offset";
    }
    $result = $this->_mysqli->query($query);
    $response = [];
    if($result){
        $response['fields'] = $this->fetchFields($result);
        $response['values'] = mysqli_fetch_all($result);
    }
    return $response;
}

/**
 * @param string $tableName
 * @param array $conditions
 * @return mixed
 */
function delete($tableName, $conditions)
{
    $whereString = $this->generateWhereString($conditions);
    $query = "DELETE FROM $tableName WHERE $whereString";
    return $query;
}

/**
 * @param array $keys
 * @param array $values
 * @return string
 */
function generateUpdateString($keys, $values)
{
    $len = count($keys);
    $buildString = '';
    for ($i = 0; $i < $len - 1; $i++) {
        $buildString .= $keys[$i] . '=' . $values[$i] . ',';
    }
    $buildString .= $keys[$len - 1] . '=' . $values[$len - 1];
    return $buildString;
}

/**
 * @param array $arrayValues
 * @return string
 */
public function generateWhereString($arrayValues)
{
    $buildString = '';
    foreach ($arrayValues as $key => $arrayValue) {
        $buildString .= $key . $arrayValue[0] . $arrayValue[1] . " " . $arrayValue[2];
    }
    return $buildString;
}

/**
 * @param string $queryResult
 * @return array
 */
function fetchFields($queryResult)
{
    if ($queryResult) {
        $fieldsData = $queryResult->fetch_fields();
        $fields = [];
        foreach ($fieldsData as $fieldData) {
            $fields[] = $fieldData->name;
        }
        return $fields;
    }
    return [];
}
}

class ORMMapper implements MapperInterface
{

private $_tableName = '';

private $_adapter;

function __construct()
{
    $this->_adapter = new MysqlAdapter('dbhost', 'username', 'password', 'test');
    if (!$this->_adapter->connect()) {
        echo "Something Terribly went wrong";
        return;
    }
    $this->loadClassProperties();
}

/**
 * @return object
 */
function findAll()
{
    $result = $this->_adapter->select($this->_tableName, '*', []);
    return $this->buildResponseObject($result);
}

/**
 * @param $id
 * @return object
 */
function findById($id)
{
    $result = $this->_adapter->select($this->_tableName, '*', ['id' => ['=', $id, '']]);
    $result = $this->buildResponseObject($result);
    if ($result){
        return $result[0];
    }
    return (object)[];
}


/**
 * @return mixed
 */
function save()
{
    // todo: Complete the Implementation of this method
    $fields = $this->_adapter->fetchFields($this->_tableName);
    if (isset($this->id)) {
        return $this->_adapter->update($this->_tableName, $fields, (array)$this, ['id' => ['=', $this->id, '']]);
    }
    return $this->_adapter->insert($this->_tableName, $fields, (array)$this);
}

function loadClassProperties()
{
    $fields = $this->_adapter->fetchFields($this->_tableName);
    foreach ($fields as $field) {
        $this->$field = null;
    }
}

/**
 * @param $result
 * @return object
 */
function buildResponseObject($result)
{
    $response = [];
    if ($result) {
        $fields = $result['fields'];
        $values = $result['values'];
        $num_of_rows = count($result['values']);
        $num_of_fields = count($result['values'][0]);

        $buildResponse = [];
        for ($i = 0; $i < $num_of_rows; $i++) {
            for ($j = 0; $j < $num_of_fields; $j++) {
                $buildResponse[$fields[$j]] = $values[$i][$j];
            }
            $response[] = $buildResponse;
        }
    }
    return json_decode(json_encode($response));
}


/**
 * @param $tableName
 */
public function setTableName($tableName)
{
    $this->_tableName = $tableName;
}
}

class TestModel extends ORMMapper{
private $tableName = 'users';
function __construct()
{
    parent::__construct();
    parent::setTableName($this->tableName);
}
}

$users = new TestModel();
$users = $users->findAll();

var_dump($users);

 array(3) { [0]=> object(stdClass)#4 (3) { ["id"]=> string(1) "1" ["i"]=> NULL ["second"]=> NULL } [1]=> object(stdClass)#7 (3) { ["id"]=> string(1) "2" ["i"]=> NULL ["second"]=> NULL } [2]=> object(stdClass)#6 (3) { ["id"]=> string(1) "3" ["i"]=> string(1) "1" ["second"]=> string(1) "2" } }