code: public function isDiagonal()
      {
          $int_size = min((array) $this->size);
  
          if($int_size > 0){
              for($i = 0; $i < $int_size; $i++){
                  $arr_row = $this->getRow($i);
  
                  if($arr_row[$i] != 0){
                      unset($arr_row[$i]);
  
                      foreach($arr_row as $v){
                          if($v != 0){
                              return false;
                          }
                      }
  
                  } else {
                      return false;
                  }
              }
  
              return true;
          }
  
          return false;
      }
docs: Tests whether the current matrix is diagonal or not.
  
  @access public
  @return boolean