You can use either system() or exec(). Just remember to ALWAYS use the full path to the command. So instead of entering 'top -n1' use '/usr/bin/top -n1'. So may also need to provide additional paramater to the command so it will run correctly as a background process. Using 'top' command as example again you need to add the '-b' parameter to make it run correctly like this '/usr/bin/top -b -n1'
When you use system() it will print all the command output to the browser. If you simply want to execute a command and you don't want the result to be displayed just redirect the output to some file like this :
system('/usr/bin/top -b -n1 > /dev/null 2>&1')
If you want to get the output (for further processing etc) use exec(). For example :
exec('/usr/bin/top -b -n1', $result)
$result will become an array containing every line of output from the 'top' command.