Save user activity to log file when post is saved

functions.php

<?php

/**
 * Function to save activity to log file when post is saved
 *
 * @param int $post_id Saved post id
 */
function motivast_add_activity_log_on_post_save( $post_id ) {

    $format = '[%s] Post with ID %d was updated by user %s from IP %s';

    $log_dir = motivast_get_logs_dir();
    $log     = $log_dir . '/.debug.log';

    $ip           = (isset($_SERVER['HTTP_CLIENT_IP'])) ? $_SERVER['HTTP_CLIENT_IP'] : ((isset($_SERVER['HTTP_X_FORWARDE‌​D_FOR'])) ? $_SERVER['HTTP_X_FORWARDED_FOR'] : $_SERVER['REMOTE_ADDR']);
    $current_user = wp_get_current_user();
    $stack_trace  = motivast_generate_stack_trace();

    $action = sprintf($format, date('Y-m-j H:i:s'), $post_id, $current_user->user_login, $ip);

    file_put_contents($log, $action, FILE_APPEND);
    file_put_contents($log, $stack_trace, FILE_APPEND);
}

add_action( 'save_post', 'motivast_add_activity_log_on_post_save' );

/**
 * Get logs directory
 * 
 * If directory does not exists create one.
 *
 * @return string
 */
function motivast_get_logs_dir() {

    $log_dir = WP_CONTENT_DIR . '/logs';

    if( !file_exists($log_dir) || !is_dir($log_dir) ) {
        mkdir($log_dir, 0755);
    }

    return $log_dir;
}

// Function from php.net http://php.net/manual/en/function.debug-backtrace.php#112238
function motivast_generate_stack_trace() {

    $e = new \Exception();

    $trace = explode( "\n" , $e->getTraceAsString() );

    // reverse array to make steps line up chronologically

    $trace = array_reverse($trace);

    array_shift($trace); // remove {main}
    array_pop($trace); // remove call to this method

    $length = count($trace);
    $result = array();

    for ($i = 0; $i < $length; $i++) {
        $result[] = ($i + 1)  . ')' . substr($trace[$i], strpos($trace[$i], ' ')); // replace '#someNum' with '$i)', set the right ordering
    }

    $result = implode("\n", $result);
    $result = "\n" . $result . "\n";

    return $result;
}