How does "do something OR DIE()" work in PHP?

Question

I'm writing a php app to access a MySQL database, and on a tutorial, it says something of the form

mysql_connect($host, $user, $pass) or die("could not connect"); 

How does PHP know that the function failed so that it runs the die part? I guess I'm asking how the "or" part of it works. I don't think I've seen it before.

Answer 1

If the first statement returns true, then the entire statement must be true therefore the second part is never executed.

For example:

$x = 5; true or $x++; echo $x;  // 5  false or $x++; echo $x; // 6 

Therefore, if your query is unsuccessful, it will evaluate the die() statement and end the script.

Answer 2

PHP's or works like C's || (which incidentally is also supported by PHP - or just looks nicer and has different operator precedence - see this page).

It's known as a short-circuit operator because it will skip any evaluations once it has enough information to decide the final value.

In your example, if mysql_connect() returns TRUE, then PHP already knows that the whole statement will evaluate to TRUE no matter what die() evalutes to, and hence die() isn't evaluated.

If mysql_connect() returns FALSE, PHP doesn't know whether the whole statement will evaluate to TRUE or FALSE so it goes on and tries to evalute die() - ending the script in the process.

It's just a nice trick that takes advantage of the way or works.