Output menu navigation in php file

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You would require the path to wp-blog-header.php to access WordPress functionality in a
standalone PHP file. So your justthemenu.php file should contain the following code.

<?php
    require('/wp-blog-header.php');
    wp_nav_menu( array( 'container_class' => 'menu-header', 'theme_location' => 'primary' ) );
?>