I recently ran across an issue that I was previously unaware of, so other developers could run into it as well. I was having problems with a plugin, which started misbehaving. The plugin had previously worked fine (it generates a sidebar widget), and I wasn’t actively working on my site, so I wasn’t really sure when it had quit working.

In the course of debugging the problem, I discovered that the plugin was throwing warnings in my PHP error log regarding the mysql_real_escape_string() function. As a quick fix, I simply replaced all of those calls with WordPress’ esc_sql() function. Voila, problem fixed.

Curious, I took a peek into wp-db.php, and found this block of code:

/* Use ext/mysqli if it exists and:
 *  - USE_EXT_MYSQL is defined as false, or
 *  - We are a development version of WordPress, or
 *  - We are running PHP 5.5 or greater, or
 *  - ext/mysql is not loaded.
 */
if ( function_exists( 'mysqli_connect' ) ) {
        if ( defined( 'USE_EXT_MYSQL' ) ) {
                $this->use_mysqli = ! USE_EXT_MYSQL;
        } elseif ( version_compare( phpversion(), '5.5', '>=' ) || ! function_exists( 'mysql_connect' ) ) {
                $this->use_mysqli = true;
        } elseif ( false !== strpos( $GLOBALS['wp_version'], '-' ) ) {
                $this->use_mysqli = true;
        }
}

Note the bit about using a development version of WordPress. In my case, I’m running out of svn trunk, and my server has the mysqli extension installed, so wpdb chose to use it. That’s fine.

But this exposes the fact that some plugins use functions like mysql_real_escape_string() “in the raw”, so to speak, which depends on using PHP’s mysql extension, and not mysqli. WordPress provides convenience functions like esc_sql() and $wpdb->prepare() to help abstract details like this away and protect developers against environmental differences between servers.

Hopefully this will save somebody else out there some debugging time.