How to Debug in Prestashop - Blank page problem after install Prestashop 1.5 ?

For Debug Prestahop 1.5, set the define variable _PS_MODE_DEV_  set to 'true' on line 28 in file config/defines.inc.php. 

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

Then,  refresh  website to see debug information. See below:  

Warning: base64_decode() has been disabled for security reasons
/home/pioneerm/public_html/prest/classes/Rijndael.php on line 35
  
its mean that, function base64_decode() should be enabled. Just contact hosting provider to confirm enabling this function