How do I query for posts by partial meta key?

Unfortunately you cannot perform a meta_query using a LIKE comparison on the meta_key value when using WP_Query. I’ve been down this road…

Instead you have a couple other options if you want to maintain like status relationships as post meta and not user meta and or meta in a custom table.

Option 1

• requires no modification of your meta schema
• uses wpdb class to perform a custom query

Example:

//when a user likes a post...
$current_user_id = get_current_user_id();
add_post_meta($current_user_id, "like_status_{$current_user_id}", 1, false);

//later in the request...
global $wpdb;

$results = $wpdb->get_results(
    "
    SELECT meta_key 
    FROM {$wpdb->prefix}postmeta 
    WHERE meta_key 
    LIKE 'like_status_%'
    ",
    ARRAY_N
);

$results = array_map(function($value){

    return (int) str_replace('like_status_', '', $value[0]);

}, $results);

array_walk($results, function($notify_user_id, $key){

    //apply to all users except the user who just liked the post
    if ( $notify_user_id !== $current_user_id ) {
        //notify logic here...           
    }

});

_{Note: logic could be simplified further if you wish.}

Option 2

• requires you change your meta schema
• requires you store the user id as the meta value
• allows you to use WP_Query along with meta_query

Option 2 requires that you change your meta key from like_status_{user_id} to something universal such as like_status or liked_by_user_id where in turn instead of storing the value of 1 against the key, you instead store the user’s id as the value.

//when a user likes a post...
$current_user_id = get_current_user_id();
add_post_meta($current_user_id, "liked_by_user_id", $current_user_id, false);

//later in the request
$args = array(
    'post_type'  => 'post', //or a post type of your choosing
    'posts_per_page' => -1,
    'meta_query' => array(
        array(
            'key' => 'liked_by_user_id',
            'value' => 0,
            'type' => 'numeric'
            'compare' => '>'
        )
    )
);

$query = new WP_Query($args);   

array_walk($query->posts, function($post, $key){

    $user_ids = get_post_meta($post->ID, 'liked_by_user_id');

    array_walk($user_ids, function($notify_user_id, $key){
        
        //notify all users except the user who just like the post
        if ( $notify_user_id !== $current_user_id ) {
                
            //notify logic here...
            //get user e.g. $user = get_user_by('id', $notify_user_id);
            
        }

    });

});