Shell_exec chmod on crontab not working?

I have a line in crontab and a php script like below:

/ etc / crontab: -

*/1 * * * * root /usr/bin/php /var/www/html/xxx.php

      

        
        
        
      

    

php: -

<?php

shell_exec("sudo chmod 655 /files/to/path/xxx.wac");

?>

      

        
        
        
      

    

When I run manually

sudo / usr / bin / php / var / www / html / xxx.php

It works as it should. However, when the crontab reaches one minute, it does not execute the command in php as it should.
Note. Checked on syslog, it runs this command every minutes.