how to obtain all subclasses of a class in php

function getSubclassesOf($parent) {
    $result = array();
    foreach (get_declared_classes() as $class) {
        if (is_subclass_of($class, $parent))
            $result[] = $class;
    }
    return $result;
}

Coincidentally, this implementation is exactly the one given in the question linked to by Vadim.

Using PHP 7.4:

$children = array_filter(get_declared_classes(), fn($class) => is_subclass_of($class, MyClass::class));