Add the following to your wp-config.php file to enable php error logging via WordPress.

@ini_set('log_errors','On'); // enable or disable php error logging (use 'On' or 'Off') 
@ini_set('display_errors','Off'); // enable or disable public display of errors (use 'On' or 'Off') 
@ini_set('error_log','/public_html/path/wordpress/php-errors.log'); // path to server-writable log file