Running CLI from a web page

I have a php cli that creates a file. It works fine when I execute it from the Linux command line as "php foo > bar". I'm trying to also execute this by clicking a form button on a web page via "system('php foo > bar')". I can see that the file bar gets created but it's always empty. I've tried executing "system('php foo')" (directly to STDOUT) but get no output. I've tried "exec('php foo',$output)" but there's nothing in the array when it's done. Suggestions??? BTW, wherever I've written "php" above I've actually used the full path of the php executable.