NonStaticSelfCall

self::method() is used to call an instance method from within the same class, but the target method is not declared static. This is only valid inside a static context.

Example

<?php
class MyClass {
    public function helper(): string {
        return 'help';
    }

    public static function run(): string {
        return self::helper(); // helper() is not static
    }
}

How to fix

Declare helper() as static, or call it on an instance instead:

<?php
public static function run(): string {
    $instance = new self();
    return $instance->helper();
}