Add system log driver and use it by default

app/Core/Log/System.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Kanboard\Core\Log;
+
+/**
+ * Built-in PHP Logger
+ *
+ * @package Kanboard\Core\Log
+ * @author  Frédéric Guillot
+ */
+class System extends Base
+{
+    /**
+     * Logs with an arbitrary level.
+     *
+     * @param mixed  $level
+     * @param string $message
+     * @param array  $context
+     * @return null
+     */
+    public function log($level, $message, array $context = [])
+    {
+        error_log('['.$level.'] '.$this->interpolate($message, $context));
+    }
+}

app/ServiceProvider/LoggingProvider.php

@@ -10,6 +10,7 @@ use Kanboard\Core\Log\Stderr;
 use Kanboard\Core\Log\Stdout;
 use Kanboard\Core\Log\Syslog;
 use Kanboard\Core\Log\File;
+use Kanboard\Core\Log\System;
 
 /**
  * Class LoggingProvider
@@ -37,6 +38,9 @@ class LoggingProvider implements ServiceProviderInterface
             case 'file':
                 $driver = new File(LOG_FILE);
                 break;
+            case 'system':
+                $driver = new System();
+                break;
         }
 
         if ($driver !== null) {

app/constants.php

@@ -26,8 +26,8 @@ defined('PLUGIN_INSTALLER') or define('PLUGIN_INSTALLER', true);
 // Enable/disable debug
 defined('DEBUG') or define('DEBUG', strtolower(getenv('DEBUG')) === 'true');
 
-// Logging drivers: syslog, stdout, stderr or file
-defined('LOG_DRIVER') or define('LOG_DRIVER', '');
+// Logging drivers: syslog, stdout, stderr, system or file
+defined('LOG_DRIVER') or define('LOG_DRIVER', 'system');
 
 // Logging file
 defined('LOG_FILE') or define('LOG_FILE', DATA_DIR.DIRECTORY_SEPARATOR.'debug.log');