if($ext == ('zip' || 'png')) { echo "Is it possible ?" }

you could use in_array($ext,array('png','zip','another','more'))

see here: http://php.net/manual/en/function.in-array.php

if($ext == ('zip' || 'png')) is doing comparison in the following order -> ('zip' || 'png'), which because at least one is not null, returns TRUE. Substitute that in now, ($ext == TRUE), which I'm going to go out on a limb and guess that php is just evaluating this the same as it would ($ext), which is also true.

if ( $ext == 'zip' || $ext == 'png' ) will check what you are looking for.

You could go with a switch-case statement:

switch($ext)
{ 
  case 'png':
  case 'zip':
       // Will run for both 'png' and 'zip'
       echo "It is possible";
       break;
  default:
       echo "unknown extension!";
       break;
 }