Call to undefined function when calling a function from plugin's menu page

Hi,

I am developing a plugin for my website. When I am trying to call a function from my plugin’s menu page it says Call to undefined function though function is there.

Code looks like this:
file 1-

[ Moderator note: Code fixed, please wrap code in backticks or use the code button. ]

class abcd{
..
function admin_menu() {
       add_menu_page('ABCG', 'ABCD XYZ', 'manage_options', 'abcd', array('abcd_gui', 'options_page'));
     }
..
}

File 2-
class abcd_gui extends abcd{
  function search($a,$b,$c){
  ..
  }
  function options_page() {
  ''
  search($a,$b,$c);
  ..
  }
}

Everything else works fine but when I try to call a function like search over here. It says Call to undefined function.

What is the problem ? I am new to wordpress development, Solutions are appreciated.