If you're running WordPress.
For WordPress the first suggestion is to edit your "wp-config.php" file and change the setting for "wp_debug" from "false" to "true". More details can be found in the WordPress Codex on Debugging.
define( 'WP_DEBUG', true );
For everything else.
You can turn on "display_errors" by adding the following line to a file called ".htaccess" in the same folder as the script having issues:
php_value display_errors on
If the file ".htaccess" does not exist, you are welcome to create one for this purpose. If you're using the cPanel File Manager you may need to go into the settings and enable displaying hidden files.
For further details on changing PHP Environment Variables please see this knowledebase article: