مشکل در پروژه مارکت اندروید

<?php
if (isset($_REQUEST['action'])){
    $action = $_REQUEST['action'];
} else {
    echo "Invalid Data";
    exit;
}

if ($action=="read"){
    read();
}

/**
 *
 */
function connectToDatabase(){
    $connection=mysqli_connect("localhost", "root", "", "market_db");
    if (mysqli_connect_errno()) {
        echo "Failed to connect to MySQL: " . mysqli_connect_error();
    }

    return $connection;
}

/**
 *
 */
function read(){
    $connection = connectToDatabase();
    $result = mysqli_query($connection, "SELECT * FROM app WHERE " . $_REQUEST['filter'] . " ORDER BY " . $_REQUEST['sort']);

    $output = array();
    while($row = mysqli_fetch_array($result)) {
        $record = array();
        $record['app_id'] = $row['app_id'];
        $record['app_name'] = $row['app_name'];
        $record['app_price'] = $row['app_price'];
        $record['app_logoUrl'] = $row['app_logoUrl'];
        $record['app_ratingValue'] = $row['app_ratingValue'];
        $record['app_author'] = $row['app_author'];
        $record['app_votes'] = $row['app_votes'];
        $record['app_file'] = $row['app_file'];
        $record['app_filesize'] = $row['app_filesize'];
        $record['app_versionName'] = $row['app_versionName'];
        $record['app_installs'] = $row['app_installs'];
        $record['app_description'] = $row['app_description'];
        $record['app_phoneNumber'] = $row['app_phoneNumber'];
        $record['app_email'] = $row['app_email'];
        $output[] = $record;
    }

    #echo "<pre>";
    #print_r($output);
    #echo "</pre>";
    echo json_encode($output);
    mysqli_close($connection);
}

منم همین مشکل دارم .کسی جواب نداد؟؟؟