<?php

class display {
    public static function error(string $message) {
        ...<render message>...
    }
}

...

if ($problem) {
    display::error("Houston, we've got a problem.");
}

<?php

class display {
    public function error(string $message) {
        ...<render message>...
    }
}

...

if ($problem) {
    $d = new display;
    $d->error("Houston, we've got a problem.");
}