Add crontab command to server by PHP script

crontab_add.php

// you will see nothing here

$output = shell_exec('crontab -l');
echo "Before<br>";
var_dump($output);
echo "<hr>";

// Create a temporary file and put into our crontab command line
// I wrote an order that will work every minute

file_put_contents('/tmp/crontab.txt', $output.'1 * * * * curl http://example.com/test_1.php'.PHP_EOL);

// Should run our temporary file
echo exec('crontab /tmp/crontab.txt');

// Let's have a look our crontab job list
$output = shell_exec('crontab -l');
echo "After<br>";
var_dump($output);
echo "<hr>";
exit();

/*
* If you want to check crontab list in terminal
* must to login by www-data, follow these commands;
* su -s /bin/bash www-data
* crontab -l
*/