Executing shell command?

Ok I am having problems with a command I would like to execute through PHP. I have tried simple backtick operators on the command, passthru, exec, system all with the same results of 0 return code. Here is an example of the command I am attempting to execute if someone could provide me with some insight, examples etc. /usr/bin/dnssec-keygen -a HMAC-MD5 -b 128 -n USER key (which works from a terminal) Here is my php code (I am changing into a writable directory which works fine): chdir( $defined['keys'] ); $cmd = $defined['dnssectool'] . " -a " . $algorithm . " -b " . $key_bit . " -n USER " . $key_name; echo $cmd . "<br>"; system( '$cmd', $ret ); chdir( $defined['virpath'] );