In documentation of PHP, about operators says:
// foo () will never be called because the entire expression is short-circuited .
What does it mean to say that the whole expression is short-circuited? What happens to the script at run time?
<?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());
The short-circuit evaluation is something implemented by the compilers / interpreters to save resources and accelerate the evaluation of conditions. In a compound conditional test, if the first condition can guarantee the execution or not of the block.
For example:
$a , as the first condition is already false and the operator is a && (E): false And anything will always give false ; $b , the first condition is true and the operator is a || (OU): true OR anything will always give true >; $c and $d follow the same idea, respectively. The link cited by @rray on operator precedence is quite valid!
I add the explanation of each operator's truth table here .
And also the specific link on short circuit, found here .