What does it mean to say that an expression is short-circuited?

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In the documentation PHP, about operators says:

// foo() will never be called because all the expression short circuit.

What does it mean to say that all expression is short-circuited? What happens to the script at runtime?

<?php

// --------------------
// foo() nunca será chamada porque toda a expressão sofre curto circuito

$a = (false && foo());
$b = (true  || foo());
$c = (false and foo());
$d = (true  or  foo());
php operators

1 answer

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Short circuit evaluation is something implemented by compilers/interpreters to save resources and speed up condition assessment. In a compound conditional test, whether the first condition can ensure the execution or not of the block.

For example:

  • with the variable $a, as the first condition is already false and the operator is a && (And): Fake and anything will always give phony;
  • with the variable $b, the first condition is true and the operator is a || (OR): REAL OR ANYTHING will always give true;
  • the examples $c and $d follow the same idea, respectively.

The link quoted by @rray about operators precedence is very valid!

I add the explanation about each operator’s truth table here.

And also the specific link on short circuit, found here.

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