What does the logical OR do in following php code

Question

if($post_id === null) {
    $this->db->insert($data);
    !isset($data[$this->page_id]) || $data[$this->page_id] = NULL;

}

Answer 1

Logical operators perform short-circuit evaluation . If the first part of a logical OR is true, the whole expression is true and hence, there is no need to evaluate the second part.

What it is doing is equivalent to:

if(isset($data[$this->page_id])){
    $data[$this->page_id] = NULL
}

To give an example of how this works, from the documentation :

// foo() will never get called as those operators are short-circuit
$a = (false && foo());
$b = (true  || foo());