Resize WordPress images on the fly using core functions

Resize WordPress images dynamically

/**
 * Resize an image to specified dimensions
 * @return (string) path to product image or feature image
 */
function my_resize_image( $post_id=null, $width=null, $height=null, $crop=true ) {

    // get path to featured image
    $featured_image_id = get_post_thumbnail_id( $post_id );
    $featured_image_path = get_attached_file( $featured_image_id );

    // make sure the file exists and it's an image
    if ( file_exists( $featured_image_path ) ) {

        // Resize
        $img = wp_get_image_editor( $featured_image_path );
        if ( ! is_wp_error( $img ) ) {
            // only resize if height and width supplied
            if ( $width || $height ) {
                $img->resize( $width, $height, true );
                $wp_upload_dir = wp_upload_dir();
                $dir = wp_mkdir_p( $wp_upload_dir['path'] );
                $filename = $img->generate_filename( 'custom', $dir, NULL );
                $saved = $img->save( $filename );
                return $wp_upload_dir['url'] . '/' . $saved['file'];
            }
        }
    }
}