Problems with autoloading classes via sp_autoload_register / maybe interfering WP-specific autoloader

There is no built-in autoloader in WordPress. But other plugins or a theme might have registered an autoloader, and depending on the order of execution these are called earlier or later than yours.

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Update 21. 08. 2014 This answer is no longer what I would recommend. Do use an autoloader if you need one.

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The only way to make sure the called class is in your code’s responsibility is checking with file_exists() and testing the class name.

file_exists() is slow and its result is cached by the file system, so you cannot rely on it anyway.

I recommend not to use an autoloader. Use a plugin specific class loader instead.

Example:

/**
 * Load a class from /php/ directory.
 *
 * There is no file_exists() check to improve performance.
 *
 * @param  string  $class         Class name
 * @param  boolean $create_object Return an object or TRUE
 * @return bool|$class
 */
function t5_es_load_class( $class, $create_object = FALSE )
{
    // create the path base just once
    static $base = FALSE;

    ! $base && $base = plugin_dir_path( __FILE__ ) . 'php';
    ! class_exists( $class ) && require "$base/class.$class.php";

    return $create_object ? new $class : TRUE;
}

Now you can use this function either to just include the class …

t5_es_load_class( 'T5_Metabox_Base' );
$metabox = new T5_Metabox_Base( 'preview' );

… or create an object …

$data               = t5_es_load_class( 'T5_Property_List', TRUE );
$data->plugin_url   = plugin_dir_url( __FILE__ );
$data->plugin_path  = plugin_dir_path( __FILE__ );

Faster, more reliable, more flexible.