Add in your code:
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);
More informations can be found here: PHP: error_reporting
Or if you don't want to show all errors in your page, you are able to check your errors in Apache logs. In Ubuntu they are stored below:
/var/log/apache2/
File: error.log
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