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 <<?php
class MyPluginPlugin {

    public function __construct() {
        Loader::loadComponents($this, array("Input"));
    }

    ...

    public function upgrade($current_version) {
        // Ensure new version is greater than installed version
        if (version_compare($this->version, $current_version) < 0) {
            $this->Input->setErrors(array(
                'version' => array(
                    'invalid' => "Downgrades are not allowed."
                )
            );
            return;
        }
    }
}
?>