WordPress shortcode to easily link to Amazon search results with associate id. Includes security considerations as well as defaults to make it as simple as possible to use.

ugotsta_amazon_search_shortcode.php

<?php
// Shortcode to easily link to Amazon search results with associate id.
// Use:
//	[amazon-search search="WordPress"]WordPress[/amazon-search]
//	[amazon-search search="WordPress"]
//	[amazon-search search="WordPress" id="my-id"]WordPress Search on Amazon[/amazon-search]
function ugotsta_amazon_search_shortcode($search, $content = null ) {

	// get attributes
	extract(shortcode_atts(array(
		'search' => 'flying+screaming+monkey',
		'id' => 'ugotsta-20'
	), $search));
	
	// escape the attributes for security purposes
	$id = esc_attr($id);
	$search = esc_attr($search);
	
	// set default content if none is provided
	if ( is_null($content) || empty($content) ) {
		// set default content if no content or search string provided
		if ( is_null($search) || empty($search) ) {
			$content = 'LINK';
		} else {
			// set default content to search string if provided
			$content = $search;
		}
	}
	// escape $content for security purposes
	$content = esc_attr($content);
	
	// convert $search to lowercase
	$search = strtolower($search);
	// convert whitespaces and underscore to plus
	$search = preg_replace("/[\s_]/", "+", $search);
	
	// setup variables for return string
	$amazon_search_url = 'http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=';	
	
	// setup link url
	$amazon_search_url = $amazon_search_url . $search . '&amp;tag=' . $id;
	
	// return full link
	return '<a href="' . $amazon_search_url . '" target="_blank">' . $content . '</a>';
}

add_shortcode('amazon-search', 'ugotsta_amazon_search_shortcode');
?>