The PHP interface keyword is used to create interfaces, which is a structure which defines a list of methods that must exist in a class.

<?php
interface my_interface {
    public function activate();
    public function deactivate();
    public function is_active();
}

class my_class implements my_interface {
    private $is_on = false;

    public function activate() {
        $this->is_on = true;
    }

    public function deactivate() {
        $this->is_on = false;
    }

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

$my_interface = new my_class();
$my_interface->activate();

if($my_interface->is_active()) {
    echo 'my_interface is on';
}
else {
    echo 'my_interface is off';
}

echo '<br>';

$my_interface->deactivate();

if($my_interface->is_active()) {
    echo 'my_interface is on';
}
else {
    echo 'my_interface is off';
}
?>