PHP currently will not log errors produced from the command line.
I have :
log_errors = On error_log = /var/log/php_errors.log
in
as a diagnostic you can attempt to force a write to the error log this way.
php -c /etc/php5/cli/php.ini -r " error_log('test 123'); "
you should now see test 123 in your log
tail /var/log/php_errors.log