Update for PHP 7 (thanks shock_gone_wild)
PHP 7 introduces the so called null coalescing operator which simplifies the below statements to:
$var = $var ?? "default";
Before PHP 7
No, there is no special operator or special syntax for this. However, you could use the ternary operator:
$var = isset($var) ? $var : "default";
Or like this:
isset($var) ?: $var = 'default';