If PHP is not reporting errors within an application (say for example a built-in error handler, or suppression), the following can be added to Apache’s vhost file:
php_admin_value error_reporting 6135 php_admin_value display_errors 1
However, if you do not have access to the vhost, the following lines can be added to a .htaccess file within the script’s location:
php_value error_reporting 6135 php_value display_errors 1
The vhost one is preferable since it will force the options to be set, and nothing can change them.