code: public function plusEquals(...$args)
      {
          if (count($args) > 0) {
              $match = implode(',', array_map('gettype', $args));
  
              switch ($match) {
                  case 'object':
                      if ($args[0] instanceof self) {
                          $M = $args[0];
                      } else {
                          throw new CalculationException(self::ARGUMENT_TYPE_EXCEPTION);
                      }
  
                      break;
                  case 'array':
                      $M = new self($args[0]);
  
                      break;
                  default:
                      throw new CalculationException(self::POLYMORPHIC_ARGUMENT_EXCEPTION);
  
                      break;
              }
              $this->checkMatrixDimensions($M);
              for ($i = 0; $i < $this->m; ++$i) {
                  for ($j = 0; $j < $this->n; ++$j) {
                      $validValues = true;
                      $value = $M->get($i, $j);
                      if ((is_string($this->A[$i][$j])) && (strlen($this->A[$i][$j]) > 0) && (!is_numeric($this->A[$i][$j]))) {
                          $this->A[$i][$j] = trim($this->A[$i][$j], '"');
                          $validValues &= StringHelper::convertToNumberIfFraction($this->A[$i][$j]);
                      }
                      if ((is_string($value)) && (strlen($value) > 0) && (!is_numeric($value))) {
                          $value = trim($value, '"');
                          $validValues &= StringHelper::convertToNumberIfFraction($value);
                      }
                      if ($validValues) {
                          $this->A[$i][$j] += $value;
                      } else {
                          $this->A[$i][$j] = Functions::NAN();
                      }
                  }
              }
  
              return $this;
          }
  
          throw new CalculationException(self::POLYMORPHIC_ARGUMENT_EXCEPTION);
      }
docs: plusEquals.
  
  A = A + B
  
  @param mixed $B Matrix/Array
  
  @return Matrix Sum