When I run this code via ssh echo wget http://domain.com/send_me_email.php | at 12:54 it run correctly and send me an email at that time. but if I run a php Like this

exec("echo wget http://domain.com/send_me_email.php | at 12:54");
exec("atq",$arr);
print_r($arr);

result of that code is something like this : job 63 at 2011-11-27 12:54 ,As you can see the job created successfully but I don't receive any Email at that time?! I test this line in php exec("wget http://domain.com/send_me_email.php"); and it send me an email, it means that I have permission to run exec and wget via php.but what is problem? I cant understand what is my problem. Please help me. thanks