CSV read and write

Csv.php

<?php
/*
 * Usage [PHP >= 7.1]
 *
require_once __DIR__ . '/Csv.php';
$csv = new CsvFile(__DIR__ . '/read_csv_file.csv');
$csvRows = $csv->getRows();
...
$csv->saveRows($csvRows, __DIR__ . '/save_csv_file.csv');
*/

class Csv
{
    private string $file;
    private array $cols;

    public function __construct(string $file)
    {
        if (!file_exists($file)) {
            throw new RuntimeException(sprintf('File %s does not exist', $file));
        }
        $this->file = $file;
    }

    public function getRows(int $length = 0, string $separator = ',', string $enclosure = '"'): array
    {
        $fp = fopen($this->file, 'rb');

        $cols = $result = [];
        $i = 0;
        while ($row = fgetcsv($fp, $length, $separator, $enclosure)) {
            if ($i === 0) {
                foreach ($row as $col => $name) {
                    $this->cols[] = $name;
                    $cols[$col] = $this->convertColName($name);
                }
            } else {
                foreach ($row as $col => $value) {
                    $result[$i - 1][$cols[$col]] = trim($value);
                }
            }
            $i++;
        }
        fclose($fp);

        return $result;
    }

    public function saveRows(array $data, string $file = null, bool $columns = true, string $separator = ',', string $enclosure = '"'): void
    {
        if (!$file) {
            $file = $this->file;
        }

        $fp = fopen($file, 'wb');
        if ($columns) {
            fputcsv($fp, $this->cols, $separator, $enclosure);
        }
        foreach ($data as $row) {
            fputcsv($fp, $row, $separator, $enclosure);
        }
        fclose($fp);
    }

    public function getDataMapping(int $asKey, array $asValue): array
    {
        $result = [];
        $csvData = $this->getRows($this->file);
        $keyName = $this->getKeyByIndex(reset($csvData), $asKey);
        foreach ($csvData as $item) {
            if (isset($item[$keyName])) {
                $values = [];
                foreach ($asValue as $val) {
                    $keyNameInValue = $this->getKeyByIndex(reset($csvData), $val);
                    if (isset($item[$keyNameInValue])) {
                        $values[] = $item[$keyNameInValue];
                    }
                }
                $result[$item[$keyName]] = count($values) > 1 ? $values : reset($values);
            }
        }
        return $result;
    }

    private function getKeyByIndex(array $row, int $key): string
    {
        $keys = array_keys($row);
        if (isset($keys[$key])) {
            return $keys[$key];
        }
        throw new OutOfRangeException(sprintf('Out on range key #%d in array', $key));
    }

    private function convertColName(string $col): string
    {
        return strtolower(trim(preg_replace('/[^A-Za-z0-9-]+/', '_', $col)));
    }
}