Conditionally assigning PHP values

For the very common case of assigning a value to a variable based on the outcome of an expression I'm a fan of ternary operators:

$foo = $bar ? $a : b;

However, if $bar is a relatively expensive operation and I want to assign the result of $bar to $foo if the result is truthy, this is inefficient:

$foo = SomeClass::bigQuery() ? SomeClass::bigQuery() : new EmptySet();

One option is:

$foo = ($result = SomeClass::bigQuery()) ? $result : new EmptySet();

But I'd rather not have the extra $result sitting in memory.

The best option I've got is:

$foo = ($foo = SomeClass::bigQuery()) ? $foo : new EmptySet();

Or, without ternary operators:

if(!$foo = SomeClass::bigQuery()) $foo = new EmptySet();

Or, if program flow operators are not your style:

($foo = SomeClass::bigQuery()) || ($foo = new EmptySet());

So many options, non of them really satisfactory. Which would you use, and am I missing something really obvious here?