code: public static function handle($level, $message, $file, $line)
      {
          // error code is not included in error_reporting
          if (!(error_reporting() & $level)) {
              return;
          }
  
          if (filter_var(ini_get('xdebug.scream'), FILTER_VALIDATE_BOOLEAN)) {
              $message .= "\n\nWarning: You have xdebug.scream enabled, the warning above may be".
              "\na legitimately suppressed error that you were not supposed to see.";
          }
  
          if ($level !== E_DEPRECATED && $level !== E_USER_DEPRECATED) {
              throw new \ErrorException($message, 0, $level, $file, $line);
          }
  
          if (self::$io) {
              self::$io->writeError('<warning>Deprecation Notice: '.$message.' in '.$file.':'.$line.'</warning>');
              if (self::$io->isVerbose()) {
                  self::$io->writeError('<warning>Stack trace:</warning>');
                  self::$io->writeError(array_filter(array_map(function ($a) {
                      if (isset($a['line'], $a['file'])) {
                          return '<warning> '.$a['file'].':'.$a['line'].'</warning>';
                      }
  
                      return null;
                  }, array_slice(debug_backtrace(), 2))));
              }
          }
      }
docs: Error handler
  
  @param int    $level   Level of the error raised
  @param string $message Error message
  @param string $file    Filename that the error was raised in
  @param int    $line    Line number the error was raised at
  
  @static
  @throws \ErrorException