Tidy from PHP command line

Hi folks,

I hope this is not an inappropriate question for this list - but I'm not 
sure where else to ask !!

I'm trying to interface Tidy from a PHP script but I just can't get it 
working. I'm trying to send a file (temp.html) to Tidy and then have 
Tidy resave it. I have this line in my PHP script.

exec(/usr/local/bin/tidy -config /usr/www/rte/tidy_config.txt -m temp.html);

What am I doing wrong folks. I've been trying for days but no success at 
all.

TIA

Huw