WP - Get posts that don't have a particular meta key

<?php
/*
    $meta_key    - What meta key to check against (required)
    $post_type   - What post type to check (default - post)
    $post_status - What post status to check (default - publish)
    $fields      - Whether to query all the post table columns, or just a select one ... all, titles, ids, or guids (all returns an array of objects, others return an array of values)
*/
function posts_without_meta( $meta_key = '', $post_type = 'post', $post_status = 'publish', $fields = 'all' ) {
    global $wpdb;

    if( !isset( $meta_key ) || !isset( $post_type ) || !isset( $post_status ) || !isset( $fields ) )
        return false;

    // Meta key is required
    if( empty( $meta_key ) )
        return false;

    // All parameters are expected to be strings
    if( !is_string( $meta_key ) || !is_string( $post_type ) || !is_string( $post_status ) || !is_string( $fields ) )
        return false;

    if( empty( $post_type ) )
        $post_type = 'post';

    if( empty( $post_status ) )
        $post_status = 'publish';

    if( empty( $fields ) )
        $fields = 'all';

    // Since all parameters are strings, bind them into one for a cheaper preg match (rather then doing one for each)
    $possibly_unsafe_text = $meta_key . $post_type . $post_status . $fields;

    // Simply die if anything not a letter, number, underscore or hyphen is present
    if( preg_match( '/([^a-zA-Z0-9_-]+)/', $possibly_unsafe_text ) ) {
        wp_die( 'Invalid characters present in call to function (valid chars are a-z, 0-9, A-Z, underscores and hyphens).' );
        exit;
    }

    switch( $fields ) :
        case 'ids':
            $cols = 'p.ID';
            break;
        case 'titles':
            $cols = 'p.post_title';
            break;
        case 'guids':
            $cols = 'p.guid';
            break;
        case 'all':
        default:
            $cols = 'p.*';
            break;
    endswitch;

    if( 'all' == $fields )
        $result = $wpdb->get_results( $wpdb->prepare( "
            SELECT $cols FROM {$wpdb->posts} p
            WHERE NOT EXISTS
            (
                SELECT pm.* FROM {$wpdb->postmeta} pm
                WHERE p.ID = pm.post_id
                AND pm.meta_key = '%s'
            )
            AND p.post_type = '%s'
            AND p.post_status = '%s'
            ",
            $meta_key,
            $post_type,
            $post_status
        ) );
    // get_col is nicer for single column selection (less data to traverse)
    else
        $result = $wpdb->get_col( $wpdb->prepare( "
            SELECT $cols FROM {$wpdb->posts} p
            WHERE NOT EXISTS
            (
                SELECT pm.* FROM {$wpdb->postmeta} pm
                WHERE p.ID = pm.post_id
                AND pm.meta_key = '%s'
            )
            AND p.post_type = '%s'
            AND p.post_status = '%s'
            ",
            $meta_key,
            $post_type,
            $post_status
        ) );

    return $result;
}
/*
    Example - fetch posts with given meta, then pass to get_posts
    -----------------
    <?php
    // Find posts without a meta entry for the fruit custom field
    $posts_without_fruit = posts_without_meta( 'fruit', '', '', 'ids' );

    // If the result didn't come back false
    if( $posts_without_fruit )

        // Pass the IDs returned into get_posts
        $posts_without_meta = get_posts( 'include=' . implode( ',', $posts_without_fruit) );

    // Basic get_posts loop
    foreach( $posts_without_meta as $post ) :
        setup_postdata( $post );

        the_title();
        echo '<br />';

    endforeach;

    wp_reset_query();
    ?>
*/
?>