Why if/else vs. or/exit in PHP?

Question

Seeing the exit() PHP documentation got me thinking:

$filename = '/path/to/data-file';
$file = fopen($filename, 'r')
or exit("unable to open file ($filename)");

Couple questions:

1. What are common use cases besides opening files for using exit()?
2. Since not every function everyone ever writes ends in exit(), how do you know to use it in some contexts vs. others?
3. Are if/else and or/exit interchangeable?

Answer 1

In that context, the or in that statement is one of PHP's logical operators which when used like that, will execute the second statement if and only if the first one fails due to short circuit evaluation.

Since fopen returned false, the or exit statement gets executed since the first part failed.

To understand it better, here is a quick explanation of short-circuit evaluation.

$x = 5;
$y = 42;

if ($x == 5 or $y == 42) {
    echo "x or y is true";
}

In the above code, the expression $y == 42 is never evaluated because there is no need since the first expression was true.

In that example, they are using the same logic for deciding whether or not to evaluate the statement that calls exit.

To address your questions:

1. I wouldn't use exit when opening a file failed unless the program was very specific. The better thing to do would be to log an error and then return the error to the caller so they can decide what to do.
2. When to use exit completely depends on the code you are writing.
3. Given the explanation about short-circuiting, yes they are interchangeable in that sense. Using or exit is just a bit shorter than using if/else.

Hope that helps.

Answer 2

1. CLI scripts, exit can take an integer parameter which is fed back to the console to indicate success or some form of error
2. I'm not inclined to use exit() or die() in application code, since exceptions are preferred. However, I personally think you might be overcomplicating things a little bit... it kills script execution, so use it when you need to kill a script. Truthfully I mostly only ever kill scripts mid-execution when debugging (one-off breakpoints) and that's not ideal either (again exceptions do a better job).
3. The use of or is mostly convenient. Here's an interesting point though...

Why does

$resource = mysql_connect() || die('dead') 

not work?

The answer is that the = operator takes precedence over or so that the assignment is made first like so: ($resource = mysql_connect()) or die(). In this way its exactly like doing an if(!($resource = mysql_connnect())) { die() }