Countable Interface

The Countable interface in PHP is a built-in interface that allows objects to be counted using the count() function.

By implementing the Countable interface, a class can define custom logic for determining its count.

<?php
class Collection implements Countable
{
    private $list = [];

    public function add($x) {
        $this->list[] = $x;
    }

    public function count()
    {
        return $this->count;
    }

}

$count = new Counter;
$count->inc();
$count->inc();


print count($count).PHP_EOL;

?>

Documentation

See also Using the count() Function in PHP

Related : count()