WordPress Image resize on the fly

resize-image.php

/**
 * Resize image on the fly
 *
 * @param  int $attachment_id Attachment ID
 * @param  int $width Width
 * @param  int $height Height
 * @param  boolean $crop Crop or not
 *
 * @return string|bool            URL of resized image, original file if error
 */
function rw_resize( $attachment_id, $width, $height, $crop = true ) {
    // Get upload directory info
    $upload_info = wp_upload_dir();
    $upload_dir  = $upload_info['basedir'];
    $upload_url  = $upload_info['baseurl'];

    // Get file path info
    $path      = get_attached_file( $attachment_id );
    $path_info = pathinfo( $path );
    $rel_path  = str_replace( $upload_dir, '', $path_info['dirname'] );

    // Generate new size
    $resized = image_make_intermediate_size( $path, $width, $height, $crop );

    if ( $resized ) {
        return "{$upload_url}{$rel_path}/{$resized['file']}";
    } else {
        // return original if fails
        return "{$upload_url}{$rel_path}/{$path_info['basename']}";
    }
}