If you want to read how to activate error reporting in Prestashop 1.5.3 this article is exactly for you. We must say, that prestashop constantly & dynamically grows up. It mean that with any new released version new features are available. Prestashop in version 1.5.3 is a bit different than earlier versions. Turning on error log is also different. 

if you want to read how to turn error log in earlier versions of prestashop - read old tutorial

What exactly is error log or error reporting?

If you are running into a white shop page or some other PHP error, if something doesn't work properly in your shop based on PrestaShop engine, if something is wrong with any presta module or addon - but you have no clue what is wrong, you should consider turning on PHP error reporting. This will give you some indication of where or what the problem is. This is a good first step to solving your problem. In simple words is a web server mode which provides you and other users to read about errors, warnings, notices in your website code. By default, in PrestaShop this feature is disabled for safety reasons. Here is how to turn on PHP errors in shops based on PrestaShop:

Open the config/defines.inc.php

Open this file and you are looking for something like I posted below:

/* Debug only */
define('_PS_MODE_DEV_',false);

you have to change it to:

/* Debug only */
define('_PS_MODE_DEV_',true);