PHP currently will not log errors produced from the command line.
I have :
log_errors = On
error_log = /var/log/php_errors.log
in
The logging/reporting behaviour of PHP is dependant on error_reporting too.
Some PHP frameworks (for example CodeIgniter) execute an error_reporting(E_STRICT) statement or equivalent, when in production mode, which will severely reduce the number/kind of logged errors.
If you want to debug something, then you can just put the following statement right before your code:
error_reporting(E_ALL);