PHP Crontab shell command utilizer class

Crontab.php

<?php

/**
 * Class Crontab
 * About: crontab shell command utilizer.
 * Author: Burak BOZ
 * GitHub: https://github.com/BurakBoz
 * License: No License & Restrictions
 */
class Crontab
{
    /**
     * @param string $jobs
     * @return array
     */
    static private function stringToArray($jobs = '')
    {
        return array_filter(explode("\n", trim(preg_replace("/(\r+|\n+)/", "\n", $jobs))));
    }

    /**
     * @param array $jobs
     * @return string
     */
    static private function arrayToString($jobs = [])
    {
        return implode("\n", $jobs);
    }

    /**
     * @param array $jobs
     * @return string|null
     */
    static private function saveJobs($jobs = [])
    {
        return shell_exec('echo ' . escapeshellarg(self::arrayToString($jobs)) . ' | crontab -');
    }

    /**
     * @return array
     */
    static public function getJobs()
    {
        return self::stringToArray(shell_exec('crontab -l'));
    }

    /**
     * @param string $job
     * @return bool
     */
    static public function doesJobExist($job = '')
    {
        return in_array($job, self::getJobs());
    }

    /**
     * @param string $job
     * @return bool|string|null
     */
    static public function addJob($job = '')
    {
        if (self::doesJobExist($job)) return false;
        $jobs   = self::getJobs();
        $jobs[] = $job;
        return self::saveJobs($jobs);
    }

    /**
     * @param string $job
     * @return bool|string|null
     */
    static public function removeJob($job = '')
    {
        if (!self::doesJobExist($job)) return false;
        $jobs = self::getJobs();
        unset($jobs[array_search($job, $jobs)]);
        return self::saveJobs($jobs);
    }
}