Implicit
A feature is implicit when it is available, yet not explicitly visible.
For example, objects are passed by reference to functions: any change applied to that object, inside the function, is also available in the calling context.
Until PHP 8.4, it was possible to assign null as a default value to any typed parameter: null was implicitly an accepted type for that value.
The contrary to implicit is explicit. This notion shares similarities with hidden features and collateral features.
<?php
function foo($object) {
$object->p = 1;
}
$object = new stdClass();
foo($object);
echo $object->p;
?>