Function to track visitor information (e. g. pixel tracker)

pixel_tracker.php

// callback
function get_server_info($k) {
    return '"' . (isset($_SERVER[$k]) ? $_SERVER[$k] : '-') . '"';
}

// format: 2016/01/06-15:38:40 10.0.0.1 "/p.gif?page=home" "Mozilla/5.0" "http://ref.co"
function track_visitor() {
    if (!isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
        $client_ip = $_SERVER['REMOTE_ADDR'];
    } else {
        $client_ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
    };

    $client_infos = array(
        'REQUEST_URI', 'HTTP_USER_AGENT', 'HTTP_REFERER'
    );
    
    $client = array_map('get_server_info', $client_infos);
    $data = array(
        date('Y/m/d-H:i:s', time()),
        $client_ip
    );
    
    $log_info = array_merge($data, $client);
    
    $log_str = implode(' ' , $log_info) . "\n";
    @file_put_contents('./pixel.txt', $log_str, FILE_APPEND);
}