* * * * * /usr/bin/php /full/path/to/script.php arg1 arg2 > /full/path/to/logfile.log
It looks like you are searching for the log file in the wrong folder. Try this
* * * * * cd /path/to/script.php ; ./script.php arg1 arg2 >> logfile.log
Then look for your log file in the /path/to/script folder. It can also be a write permission problem. Also, check your script for errors. Your crontab command seems OK.