Fatal error: Cannot access property CI_Exceptions::$ob_level in /var/www/html/system/

Please help me to investigate this error.
I have same code are deploy on 2 servers . 
Server1 cant show the right error but Server2 is display Fatal error and can not display actual error.

Error on Server1
   

Error on Server2
   

// This is code.

ini_set('display_errors', 1);
error_reporting(-1);

printf("INDEX");
  $a = $x["a"];
printf("INDEX2");