PHPOffice / PhpSpreadsheet

                            for ($k = 0; $k < $this->n; ++$k) {

                                $Bcolj[$k] = $B->A[$k][$j];

                            }

                            for ($i = 0; $i < $this->m; ++$i) {

                                $Arowi = $this->A[$i];

                                $s = 0;

                                for ($k = 0; $k < $this->n; ++$k) {

                                    $s += $Arowi[$k] * $Bcolj[$k];

                                }

                                $C->A[$i][$j] = $s;

                            }

                        }

                        return $C;

                    if ($this->n == $B->m) {

                        $C = new self($this->m, $B->n);

                        for ($i = 0; $i < $C->m; ++$i) {

                            for ($j = 0; $j < $C->n; ++$j) {

                                $s = '0';

                                for ($k = 0; $k < $C->n; ++$k) {

                                    $s += $this->A[$i][$k] * $B->A[$k][$j];

                                }

                                $C->A[$i][$j] = $s;

                            }

                        }

                        return $C;