Not Operator
The not operator return the opposite boolean of a value. The value is first converted to boolean, according to classic PHP rules, then, negated. This operator returns a boolean.
There is also a bitwise not operator, which works at the bit level, on the value: ~.
<?php
$a = 3;
if (!$a) {
print 'OK';
} else {
print 'KO';
}
?>