Binary Not Operator

The tilde ~ operator return the opposite value, bit by by. The value is first converted to an integer, then, negated. This operator returns an integer.

~ is often confused with !, as some usage quite overlap.

<?php

$a = 3;     // 3
$b = ~$a;   // -4
$c = !$a;   // false

?>

Documentation

Related : Not Operator, ~ Tilde