fiddle_091187.php

<?php
  $condition = true;
  
  $arr = array(
  	'a' => 'abc', 
  	($condition ? 'b' : '') => ($condition ? '123' : ''),
  	($condition ? 'c' : '') => ($condition ? 'xyz' : ''),
  );
  
  $arr = array_filter($arr);
  var_dump($arr);
?>