var a = 0 || 3;
will assign 3 to the variable. Such construct will assign boolean true in PHP.
You still can get default value on 1 line, using the fact that or has lower precedence than assignment.
$a = 0 or $a = 3;
which in a real-case scenario can look like:
$a = $b or $a = $c; // pick the first non-empty value
$a = some_call() or $a = $default;