CodeIgniterのログを独自カテゴリーに分けて出力する ref: https://qiita.com/Go-Noji/items/dbf248f2fd55ed48ca58

file0.php

log_message('debug', 'デバックメッセージ');

file3.php

log_message('debug', 'デバックメッセージ');

file4.php

log_message('custome', 'カスタムメッセージ');

log-2018-10-05.php

<?php defined('BASEPATH') OR exit('No direct script access allowed'); ?>

DEBUG - 2018-10-05 17:33:35 --> デバックメッセージ

log-custome-2018-10-05.php

<?php defined('BASEPATH') OR exit('No direct script access allowed'); ?>

DEBUG - 2018-10-05 17:36:15 --> カスタムメッセージ

MY_Log.php

class MY_Log extends CI_Log {

  private function _write_log($level, $msg)
  {
    $category = isset($this->_levels[$level]) ? '' : $level.'-';

    $filepath = $this->_log_path.'log-'.$category.date('Y-m-d').'.'.$this->_file_ext;
    $message = '';

    if ( ! file_exists($filepath))
    {
      $newfile = TRUE;
      // Only add protection to php files
      if ($this->_file_ext === 'php')
      {
        $message .= "<?php defined('BASEPATH') OR exit('No direct script access allowed'); ?>\n\n";
      }
    }

    if ( ! $fp = @fopen($filepath, 'ab'))
    {
      return FALSE;
    }

    flock($fp, LOCK_EX);

    // Instantiating DateTime with microseconds appended to initial date is needed for proper support of this format
    if (strpos($this->_date_fmt, 'u') !== FALSE)
    {
      $microtime_full = microtime(TRUE);
      $microtime_short = sprintf("%06d", ($microtime_full - floor($microtime_full)) * 1000000);
      $date = new DateTime(date('Y-m-d H:i:s.'.$microtime_short, $microtime_full));
      $date = $date->format($this->_date_fmt);
    }
    else
    {
      $date = date($this->_date_fmt);
    }

    $message .= $this->_format_line($level, $date, $msg);

    for ($written = 0, $length = self::strlen($message); $written < $length; $written += $result)
    {
      if (($result = fwrite($fp, self::substr($message, $written))) === FALSE)
      {
        break;
      }
    }

    flock($fp, LOCK_UN);
    fclose($fp);

    if (isset($newfile) && $newfile === TRUE)
    {
      chmod($filepath, $this->_file_permissions);
    }

    return is_int($result);
  }

  /**
   * Write Log File
   *
   * Generally this function will be called using the global log_message() function
   *
   * @param string  $level  The error level: 'error', 'debug' or 'info'
   * @param string  $msg    The error message
   * @return    bool
   */
  public function write_log($level, $msg)
  {
    if ($this->_enabled === FALSE)
    {
      return FALSE;
    }

    $level = strtoupper($level);

    if ( ! isset($this->_levels[$level]))
    {
      return $this->_write_log($level, $msg);
    }

    if (
      isset($this->_levels[$level])
      && (($this->_levels[$level] > $this->_threshold))
      && ! isset($this->_threshold_array[$this->_levels[$level]])
    )
    {
      return FALSE;
    }

    return $this->_write_log($level, $msg);
  }

}