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<?php
/**
 * Snippet for mass re-generating slugs in WordPress. Place at the bottom of wp-config.php. Backup your DB first.
 */

//  We prepare a variable to hold arrays with post titles so we don't accidentally make duplicates, that would be bad
$slug_done = array();

//  Run a query to grab all the posts, we only want posts/pages though
$posts = $wpdb->get_results("
    SELECT
        `ID`,
        `post_title`
    FROM
        `" . $wpdb->posts . "`
    WHERE
        `post_type` = 'page'
    OR
        `post_type` = 'post'
");

//  Loop through results
foreach($posts as $single)
{
    //  Generate a URL friendly slug from the title
    $slug_base = sanitize_title_with_dashes( $single->post_title );
    $this_slug = $slug_base;

    //  Check if the slug already exists, if it does, we add an incremental integer at the end of it
    $slug_num = 1;
    while (in_array($this_slug, $slug_done))
    {
        $this_slug = $slug_base.'-'.$slug_num;
        $slug_num++;
    }
    $slug_done[] = $this_slug;

    //  We are happy with our slug, update the database table
    $wpdb->query("
        UPDATE
            `" . $wpdb->posts . "`
        SET
            `post_name` = '" . $this_slug . "'
        WHERE
            `ID` = '" . $single->ID . "'
        LIMIT 1
    ");
}