Why are all these PHP statements written with the not (!) operator?

I subscribe to the same mindset as you're seeing in the docs, as to many other developers, and that is that we right code to first cover every possible way something might go wrong. For two reasons, one is it helps to remember all the ways something can fail by first addressing those scenarios, and only lastly addressing what to do if it was successful. The second, is many times code is not asynchronous, meaning that it runs one step at a time. This means we can be sure that our code goes through every check for failure first, before getting to the successful code part.

It's like a rocket launch. They check every system for failure, and only after they are sure that every check they threw at it did not fail, do they give the go ahead for a successful launch.

By first checking for all the failures, we can be reasonable assured our code was successful. If you check first if it was successful, you may have a lot of things you need to check for to be sure it actually was successful, which leads to messier, and possibly less dry code.

Hi Keith,

Is it correct that in these examples with the negation operator, you don't think that any database operations are being performed? In other words, the code isn't doing any actual database work here, it's only saying that error messages should be echoed if things don't work?

The database work is actually being performed in the evaluation of the if conditions.

It might help to look at an example from the link you provided.

Here's the first example with a non prepared statement:

/* Non-prepared statement */
if (!$mysqli->query("DROP TABLE IF EXISTS test") || !$mysqli->query("CREATE TABLE test(id INT)")) {
    echo "Table creation failed: (" . $mysqli->errno . ") " . $mysqli->error;
}

So the php interpreter needs to evaluate that if condition to see if it's true or false. To do that it needs to call the query method on the $mysqli object and that method will return a value back to the if condition for further evaluation.

So when the query method is called is when the actual attempt to drop the table happens. If will return true if successful and false on failure.

If it failed then you have !false which is true which means you'll execute the if block and echo an error message.

On the other hand, let's suppose it succeeded. This means it needs to evaluate the right side of the OR operator. Again, this involves calling the query method and attempting to create the table. This will either return true or false depending on the result.

If that succeeded as well then the entire if condition evaluates to false and you won't print an error message.

So when you've reached the end of the if statement, you've either printed an error message because what you tried to do failed or the database operation was successful and you skipped over the if block.

Does that help at all?