Update

data_sync.php

<?php
/**
 * Created by PhpStorm.
 * User: Omolara Adejuwon
 * Date: 18/11/2015
 * Time: 18:22
 */
require_once("./DataSyncHandler.php");
$dataSyncHandler = new DataSyncHandler();
header("Content-Type: application/json"); //set content type to application/json
if (isset($_GET["operation"])) {
    $operation = $_GET["operation"];
    switch ($operation) {
        case "compare":
            if (isset($_GET["tableName"]) && isset($_GET["ids"])) {
                $tableName = $_GET["tableName"];
                $ids = $_GET["ids"];
                header("HTTP/1.1 200 OK");
                echo json_encode(array("response_code" => 200, "data" => $dataSyncHandler->compareData($tableName, $ids)));
            } else {
                header("HTTP/1.1 400 Bad Request");
                echo json_encode(array("response_code" => 400, "message" => "Some parameters are missing"));
            }
            break;
        case "upload":
            if (isset($_GET["tableName"])) {
                $tableName = $_GET["tableName"];
                $json = file_get_contents("php://input");
                $dataArray = json_decode($json, true);
                $response = array();

                if (is_array($dataArray) || is_object($dataArray)) {
                    print_r($dataArray["data"]);
                    $response = array();
                    for ($x = 0; $x < count($dataArray["data"]); $x++) {
                        echo "<br>";
                        $response = $dataSyncHandler->getAndInsertDataFromDevice($tableName, $dataArray["data"][$x]);
                        // print_r(array_values($dataArray[$key][0]));
                    }

                }
                header("HTTP/1.1 200 Bad Request");
                echo json_encode(array("response_code" => 200, "data" => $response));
            } else {
                header("HTTP/1.1 400 Bad Request");
                echo json_encode(array("response_code" => 400, "message" => "Some parameters are missing"));
            }
            break;
        case "download":
            if (isset($_GET["tableName"]) && isset($_GET["ids"])) {
                $tableName = $_GET["tableName"];
                $ids = $_GET["ids"];
                header("HTTP/1.1 200 OK");
                echo json_encode(array("response_code" => 200, "data" => $dataSyncHandler->sendDataToDevice($tableName, $ids)));
            } else {
                header("HTTP/1.1 400 Bad Request");
                echo json_encode(array("response_code" => 400, "message" => "Some parameters are missing"));
            }
            break;
    }
} else {
    header("HTTP/1.1 400 Bad Request");
    echo json_encode(array("response_code" => 400, "message" => "Some parameters are missing"));
}

DataSyncHandler.php

<?php

/**
 * Created by PhpStorm.
 * User: Omolara Adejuwon
 * Date: 18/11/2015
 * Time: 11:10
 */
require_once("./db_function.php");

class DataSyncHandler
{
    private $tableName;
    private $db;

    public function __construct($tableName = "users")
    {
        //$this->tableName = $tableName;
        $this->db = new DB_Function();
    }

    public function compareData($tableName = "users", $idsFromDevice = "35,10,1,5,32,9")
    {
        $idsFromDeviceArray = explode(",", $idsFromDevice);
        $idResource = $this->db->getIDs($tableName);
        $idsArray = array();
        while ($row = mysqli_fetch_assoc($idResource)) {
            $idsArray[] = $row["id"];
        }
        $a = $this->flip_isset_diff($idsFromDeviceArray, $idsArray);//returns missing ids from server
        $b = $this->large_array_diff($idsFromDeviceArray, $idsArray);
        $c = $this->my_array_diff($idsFromDeviceArray, $idsArray);
        $d = $this->flip_isset_diff($idsArray, $idsFromDeviceArray); //returns missing ids from device
        $e = $this->large_array_diff($idsArray, $idsFromDeviceArray);
        $f = $this->my_array_diff($idsArray, $idsFromDeviceArray);
        return array("flip" => $a, "diff" => $b, "my_diff" => $c, "flip_device" => $d, "diff_device" => $e, "my_diff_device" => $f);
    }

    function flip_isset_diff($b, $a)
    {
        $start = microtime(true);
        $at = array_flip($a);
        $d = array();
        foreach ($b as $i)
            if (!isset($at[$i]))
                $d[] = $i;
        echo (microtime(true) - $start) . PHP_EOL;
        return $d;
    }

    function large_array_diff($b, $a)
    {
        // Flipping
        $start = microtime(true);
        $at = array_flip($a);
        $bt = array_flip($b);
        // checking
        $d = array_keys(array_diff_key($bt, $at));
        echo (microtime(true) - $start) . PHP_EOL;
        return $d;
    }

    function my_array_diff($b, $a)
    {
        $start = microtime(true);
        $d = array_values(array_diff($b, $a));
        echo (microtime(true) - $start) . PHP_EOL;
        return $d;

    }

    function getAndInsertDataFromDevice($tableName = "users", $dataFromDevice)
    {
        $columns = implode(", ", array_keys($dataFromDevice));
        $escaped_values = array_map(array($this, 'escape'), $dataFromDevice);
        $values = "'" . implode("','", $escaped_values) . "'";
        return $this->db->insertData($tableName, $columns, $values);
    }

    function escape($array)
    {
        return mysql_real_escape_string($this->db->mysqli, $array);
    }

    function sendDataToDevice($tableName = "users", $idsFromDevice = "12,26,29")
    {
        $idResource = $this->db->getDataWhereID($tableName, $idsFromDevice);
        $idsArray = array();
        while ($row = mysqli_fetch_assoc($idResource)) {
            $idsArray[] = $row;
        }
        return $idsArray;
    }
}