PHP EXEC NOHUP

I am trying to use the php exec command it have a process continue in the background even if the user closes the window. Before this, I would run:
exec("./a.out > /dev/null &") with no problem.

however, now i need run:
exec("nohup ./a.out > /dev/null &")

when i run this, i receive the following error:
/var/www/html/secure/functions/nohup: No such file or directory

how can i have exec take the entire parameter as the command, not just the word?

Thanks,
Orlando