Adds a custom image url field in RSS2 output of Amazon Auto Links (WordPress plugin) product RSS2 feeds.

amazon-auto-links-custom-rss-field.php

<?php
/**
 *	Plugin Name:    Amazon Auto Links - Custom RSS ImageURL Field
 *	Plugin URI:     http://en.michaeluno.jp/amazon-auto-links
 *	Description:    Adds a custom image url field in RSS2 output of Amazon Auto Links product RSS2 feeds. Requires Amazon Auto Links 3.5.4 or above.
 *	Author URI:     http://michaeluno.jp
 *	Version:        0.0.1
 *  Author:         Michael Uno
 */
 

new AmazonAutoLinks_CustomRSS2Field( 
    "imageurl", // tag name
    "",         // the name space url e.g. http://my-namespace.sample/imageurl
    500        // image size in pixel.  default: 500,  set 0 to use the unit's image size
);

class AmazonAutoLinks_CustomRSS2Field {

    private $___sTagName = '';
    private $___sNamespaceURL = '';
    private $___iImageSize = 500;

    public function __construct( $sTagName, $sNamespaceURL, $iImageSize=500 ) {
        $this->___sTagName = $sTagName;
        $this->___sNamespaceURL = $sNamespaceURL;
        $this->___iImageSize = $iImageSize;
        add_action( 'aal_action_rss2_namespace', array( $this, '_doOnAALAction_RSS2Namespace' ) );
        add_action( 'aal_action_rss2_item', array( $this, '_doOnAALAction_RSS2Item' ) );       
    }

    public function _doOnAALAction_RSS2Namespace() {
        if ( ! $this->___sNamespaceURL || ! $this->___sTagName )  {
            return;
        }
        echo 'xmlns:' . $this->___sTagName . '="' . $this->___sNamespaceURL. '"';        
    }
    
    public function _doOnAALAction_RSS2Item( $aProduct ) {
        echo '<' . $this->___sTagName .  '>'
                . $this->___setImageSize( $aProduct[ "thumbnail_url" ], $this->___iImageSize )
            . '</' . $this->___sTagName . '>';       
    }
    
        /**
         * Returns the resized image url.
         *
         * Examples
         *  _SL160_   -> _SL500_
         *  _SS160_   -> _SS500_
         */
        private function ___setImageSize( $strImgURL, $numImageSize ) {
            if ( ! $numImageSize  )  {
                return $strImgURL;
            }
            return preg_replace( '/(?<=_S)([LS])(\d+){3}(?=_)/i', 'S${2}'. $numImageSize . '', $strImgURL );              
        }    
    
}