function foo()
{
  $foo = array();
  $foo['bar'] = 'foobar';

  return {TYPE}; // one of 5 {TYPE}
}

result of 100,000 excute : seconds

{TYPE} : one of this :

1. return 'some '.$foo['bar'].' test';  // 1.8925 // winner
2. return "some $foo[bar] test";        // 2.8418
   return "some {$foo['bar']} test";    // 2.8423
3. return 'some '.$foo[bar].' test';    // 3.2916
4. return "some {$foo[bar]} test";      // 4.5538