Load WordPress in custom PHP Script:

You need to load essential WordPress core functionality in your custom PHP script for WP_Query to work properly.

For example, let’s say you have a custom PHP file named my-cron.php and WordPress is installed in the web root, like this:

public_html/
    index.php
    my-cron.php <--
    wp-load.php
    wp-settings.php
    ...
    wp-admin/
    wp-content/
    wp-includes/

In this setup, if you want to use WP_Query in my-cron.php file, you need to load the wp-load.php file. So in my-cron.php file you need to have the following CODE:

if ( ! defined('ABSPATH') ) {
    /** Set up WordPress environment */
    require_once( dirname( __FILE__ ) . '/wp-load.php' );
}

Access WP_Query:

At this point, you’ll have the access to WP_Query, so you can use it like this:

// simply selecting posts with category name "wordpress"
$the_query = new WP_Query( array( 'category_name' => 'wordpress' ) );
if ( $the_query->have_posts() ) {
    echo '<ul>';
    while ( $the_query->have_posts() ) {
        $the_query->the_post();
        echo '<li>' . get_the_title() . '</li>';
    }
    echo '</ul>';
}
else {
    echo "No post found for category named wordpress";
}

Delete Posts:

However, WP_Query doesn’t have delete function. For that you’ll either need to use the wp_delete_post() function or WPDB class. Using wp_delete_post() is recommended as it’ll take care of many dependencies, however, if you need more control, then you may use WPDB class or $wpdb global variable, but be careful if you choose that path.

For example, the following CODE will delete the post with ID 1:

$deleted = wp_delete_post( 1 );
if( $deleted === false ) {
    echo "Couldn't delete Post with ID=1";
}
else {
    echo "Deleted Post with ID=1";  
}

Of course you can combine WP_Query with wp_delete_post to find and delete posts that meet specific criteria.

Setup Cron:

Once you are done writing the custom PHP script, you need to setup cron job to run as a HTTP request, like the following:

5 * * * * wget -q -O - http://your-domain.com/my-cron.php

Security:

Since accessing WP_Query or wp_delete_post function doesn’t require any authentication (or permission) by default, you need to make sure my-cron.php is not publicly accessible. For example, you can add the following at the beginning of my-cron.php file to give access to localhost only:

$allowed_ip = '127.0.0.1';
if( $allowed_ip !== $_SERVER['REMOTE_ADDR'] ) {
    exit( 0 );
}