Config.php

<?php

use Phinx\Config\Config as PhinxConfig;

class Config extends PhinxConfig
{
    /**
     * {@inheritdoc}
     */
    public function getMigrationPath()
    {
        // use get_template_directory, plugin_dir_path, etc...
        return '/the/path/to/your/migrations/directory';
    }

    /**
     * {@inheritdoc}
     *
     * @param boolean $dropNamespace Return the base migration class name without the namespace.
     * @return string
     */
    public function getMigrationBaseClassName($dropNamespace = true)
    {
        return 'SomeFullyQualifiedMigrationBaseClassName';
    }
}