php syntax: program flow control using the || operator [duplicate]

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PHP short circuit lazy evaluation, where is it in the php.net manual?
PHP “or” Syntax

I have seen people using the || operator as program flow control as follows:

function() || die("message");

where die("message"); will run if function() returns false. Furthermore, it seems to only work for die(); and exit(); else the interpreter will throw a "syntax error" message.

I'm wondering what this is called and where can I find documentation for its behaviour.

Answer 1

It's just a boolean OR expression. The usage is taking advantage of a behavior called short cutting, where if the first part of the expression evaluates to true, then the second half isn't evaluated because the OR expression is already true.

Answer 2

It's just logic OR. If function() returns true, then the rest of the expression is not evaluated.

Answer 3

This is due to OR / || being an operator with left precedence (see here: http://www.php.net/manual/en/language.operators.precedence.php) as the left is evaluated to be true, there is no point in evaluating the right side as the expression will always be true.