Keep in mind that setValue won't work for readonly properties.  

<?php

class Person
{
    public function __construct(private readonly int $age) {}
}

$someOldPerson = new Person(80);

$reflection = new ReflectionProperty($someOldPerson, 'age');
$reflection->setValue($someOldPerson, 10); // Fatal error: Uncaught Error: Cannot modify readonly property Person::$age

setValue can be used for readonly properties, but only if the property has not yet been initialised:

<?php

class Person
{
    private readonly int $age;
    public function __construct(array $props = []) {
        if (isset($props['age'])) {
            $this->age = (int)$props['age'];
        }
    }
}

$personWithKnownAge = new Person(['age' => 50]);

$reflection = new ReflectionProperty($personWithKnownAge, 'age');
$reflection->setValue($personWithKnownAge, 10); // Fails - Age is already initialised, value cannot be changed.

$personWithUnknownAge = new Person();

$reflection = new ReflectionProperty($personWithUnknownAge, 'age');
$reflection->setValue($personWithUnknownAge, 10); // Succeeeds - Age is not yet initialised, value can be set.
?>

This can be useful for situations where it is desirable to initialise properties from outside of the defining class, for example an ORM setup where the parent class is responsible for setting properties on a model subclass instance.

You can use ReflectionProperty::setValue to set the value on static properties as well as regular instance properties.  Simply pass null in place of the instance:

<?php
class Foo {
    protected static $bar = null;
    public static function sayBar() {
        echo self::$bar;
    }
}

$r = new ReflectionProperty('Foo', 'bar');
$r->setAccessible(true);
$r->setValue(null, 'foo');

Foo::sayBar(); // "foo"
?>