Return all users with a specific meta key

I’d like to use get_users() to return all users that have a particular meta_key, but I don’t want to specify what the value has to be because it will change for every user. The value is a json string. Is this possible?

You can use the just the meta_key argument, the result will be similar to using the EXISTS sql statement.

<?php
$users = get_users(array(
    'meta_key'     => 'your_meta_key',
));

Alternatively, you can use an empty string for meta_value (the default) and > for meta_compare. The result is the same (probably because meta_value gets ignored if its empty!).

<?php
$users = get_users(array(
    'meta_key'     => 'your_meta_key',
    'meta_value'   => '',
    'meta_compare' => '>',
));

The first method works when using WP_Query and meta_query as well. Example from a plugin of mine:

    $links = get_posts(array(
        'post_type'   => self::POST_TYPE,
        'numberposts' => apply_filters('seoal_number_links', -1),
        'meta_query'  => array(
            'relation' => 'AND',
            array(
                'key'     => self::get_key("type_{$post->post_type}"),
                'value'   => 'on',
                'compare' => '='
            ),
            array(
                'key'     => self::get_key('url'),
                'compare' => 'EXISTS' // doesn't do anything, just a reminder
            ),
            array(
                'key'     => self::get_key('keywords'),
                'compare' => 'EXISTS' // doesn't do anything, just a reminder
            )
        ),
        'suppress_filters' => false,
    ));