如何通过php脚本编辑crontab？

I have a function which writes into the crontab using PHP script:

function changejob() {
global $existingJobs;
global $newJob;
chmod("/tmp/crontab", 0764);   
if (file_put_contents('/tmp/crontab', $newJob.PHP_EOL)) {
    if (exec('crontab /tmp/crontab')) { 
        echo 'Crontab changed to ' .$newJob;    
    }
    else {
        echo 'Error with exec command';
    }
}
else {
    echo 'file_put_input function failed';
}

}

The script echoes 'Error with exec command'. If I immediately then run crontab /tmp/crontab in the command line, the crontab file is edited correctly. So I know the file_put_contents function worked and the echo exec() function did not.

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