<?php
//Change these based on what user ids your registered users have.
$user_1_id = 1;
$user_2_id = 2;

/**
 * When we create an user, let's assume we attach a secret access key to his user ID, under the meta_key 'access_key'.
 *
 * We ask the user for this key every time he wants to do something.
 */
update_user_meta( $user_1_id, 'access_key', 'eiZurewj$ez24pP' );
update_user_meta( $user_2_id, 'access_key', 'xcrpsokfoipu35oE' );

/**
 * This is the key that he provides us.
 */
$secret_key_from_frontend = 'eiZurewj$ez24pP';

$all_users_ids = get_users([
    'fields' => 'id',
    'meta_key' => 'access_key',
    'meta_compare' => '=',
    'meta_value' => $secret_key_from_frontend
]);

/**
 * We loop through a supposedly safe & accurate list of users that match both the meta_key/value pair.
 * It should be only give us '1'.
 */
echo "User ids that correspond to the correct secret key:";
//Should only return 1.
foreach( $all_users_ids as $user_id ) {
    echo $user_id;
    echo " ";

    //Do some sensitive stuff with this, since we "know" the user has the secret key for a specific user id.
}

echo "<br>";

/**
 * So, let's go ahead and mess with it by making the meta_value empty.
 */

$evil_key = '';

$evil_user_ids = get_users([
    'fields' => 'id',
    'meta_key' => 'access_key',
    'meta_compare' => '=',
    'meta_value' => $evil_key
]);

echo "User ids that correspond to the evil, empty-space key:";
//Returns 1,2...
foreach( $evil_user_ids as $evil_user_id ) {
    echo $evil_user_id;
    echo " ";

    //Do some sensitive stuff with this, only this time, we got tricked, we're doing the same operation for all users.
}


echo "However, let us see what happens when False is provided for the meta value.";
$proper_user_ids = get_users([
    'fields' => 'id',
    'meta_key' => 'access_key',
    'meta_compare' => '=',
    'meta_value' => False
]);

echo "User ids that correspond to the evil, empty-space key:";
//Returns nothing.
foreach( $proper_user_ids as $proper_user_id ) {
    echo $proper_user_id;
    echo " ";

    //Do some sensitive stuff with this, only this time, we got tricked, we're doing the same operation for all users.
}

echo "Well, nothing, as it should, but, we are expecting that '' would achieve the same thing.";