Unix `at` command via PHP

Question

Here's my code to create a new at job...
system('echo \'php -f /path/to/my/php/file.php\' | at 1700');

I thought this would be simple and would just work but alas, nothing happens!

When I run echo \'php -f /path/to/my/php/file.php\' | at 1700 via ssh everything works as expected.

Is this a permissions problem? I.e PHP isn't allowed to create new at jobs?