In case you are not able for some reasons to setup PHP error log through php.ini settings you can use this code to setup your own PHP error handler and thus write needed PHP errors/event into your own log file:

<?php phpinfo(); ?> |
Call it from the browser as http://yourBlogURL/phpinfo.php
You will see standard ‘PHP Info’ page with values of different PHP configuration parameters. Check display_errors parameter value under ‘PHP Core section’ now.