Two things you will need to do (from the command prompt):
1) Insure that PHP is indeed located @ /usr/bin/php
/usr/bin/php --version
2) Add the full path to the crontab entry:
0 7 3 * * /usr/bin/php /home/users/test.com/index.php cli/cart_reminder
Finally once you have that command set, do the following to insure it actually works:
>cd $HOME
>/usr/bin/php /home/users/name/sites/test.com/index.php cli/cart_reminder