Try getting rid of the file output completely.
/usr/bin/php -q /home/yamik/public_html/forums/Sources/shop/dointerest.php
Then, instead of writing it to a file, it will email you the output. Once you've ensured it works fine, you can discard the output completely:
/usr/bin/php -q /home/yamik/public_html/forums/Sources/shop/dointerest.php > /dev/null