Paypal Subscription Button Class allows you to easily create a subscription form or email link from the given variables.

example.php

<?php


	// initializing
	$post = get_post($atts['item_number']);
	$meta = get_post_custom($post->ID);
	
	foreach ($meta as $p => $v)
		$meta[$p] = $v[0];
	
	$datts = array(
		'item_number' => $post->ID,
		'item_name' => $post->post_title,
	);
	
	$datts = wp_parse_args($datts, $meta);
	$atts = wp_parse_args($atts, $datts);
	
	// populating the class
	$button = redrokk_ppbutton_class::getInstance($atts);
	
	if (isset($meta['a1']) && $meta['a1'][0])
	{
		$button->set('a1', $meta['a1']);
		$button->set('p1', $meta['p1']);
		$button->set('t1', $meta['t1']);
	}
	
	if (isset($meta['a2']) && $meta['a2'])
	{
		$button->set('a2', $meta['a2']);
		$button->set('p2', $meta['p2']);
		$button->set('t2', $meta['t2']);
	}

ppbutton.class.php

<?php 
/**
 * @Author	Anonymous
 * @link http://www.redrokk.com
 * @Package Wordpress
 * @SubPackage RedRokk Library
 * @copyright  Copyright (C) 2011+ Redrokk Interactive Media
 * 
 * @version 0.1
 */
defined('ABSPATH') or die('You\'re not supposed to be here.');

/**
 * Class creates a payment link for posting to paypal.
 * 
 * @see https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_Appx_websitestandard_htmlvariables
 * @author Anonymous
 * @example

	$button = redrokk_ppbutton_class::getInstance();

 */
class redrokk_ppbutton_class
{
	/**
	 * The post ID
	 * 
	 * @var int
	 */
	var $_id;
	
	/**
	 * 
	 * @var subscription
	 */
	var $cmd = '_xclick-subscriptions';
	
	/**
	 * (Required) Your PayPal ID or an email address associated with your 
	 * PayPal account. Email addresses must be confirmed.
	 * 
	 * @var string
	 */
	var $business;
	
	/**
	 * (optional) The language of the login or sign-up page that 
	 * subscribers see when they click the Subscribe button. If 
	 * unspecified, PayPal determines the language by using a cookie 
	 * in the subscriber’s browser. If there is no PayPal cookie, the 
	 * default language is U.S. English.
	 * 
	 * @see https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_country_codes
	 * 
	 * @var string
	 */
	var $lc = 'US';
	
	/**
	 * Description of item. If this variable is omitted, buyers enter their 
	 * own name during checkout.
	 * 
	 * Optional for Buy Now, Donate, Subscribe, Automatic Billing, Installment 
	 * Plan, and Add to Cart buttons. Not used with Buy Gift Certificate buttons.
	 * 
	 * @var string
	 */
	var $item_name;
	
	/**
	 * Pass-through variable for you to track product or service purchased or 
	 * the contribution made. The value you specify is passed back to you upon 
	 * payment completion. This variable is required if you want PayPal to 
	 * track inventory or track profit and loss for the item the button sells.
	 * 
	 * @var string
	 */
	var $item_number;
	
	/**
	 * (Required) Do not prompt buyers to include a note with their payments. 
	 * Allowable values for Subscribe buttons:
	 * 
	 * 	1 – hide the text box and the prompt
	 * For Subscribe buttons, always include no_note set to 1.
	 * 
	 * @var boolean
	 */
	var $no_note = 1;
	
	/**
	 * Do not prompt buyers for a shipping address.
	 * Allowable values are:
	 * 
	 * 		0 – prompt for an address, but do not require one. The default is 0.
	 * 		1 – do not prompt for an address
	 * 		2 – prompt for an address, and require one
	 * 		
	 * @var boolean
	 */
	var $no_shipping = 1;
	
	/**
	 * The URL to which PayPal redirects buyers’ browser after they complete 
	 * their payments. For example, specify a URL on your site that displays a 
	 * "Thank you for your payment" page.
	 * 
	 * Default – PayPal redirects the browser to a PayPal webpage.
	 * 
	 * @var string
	 */
	var $return_url;
	
	/**
	 * The URL to which the sender’s browser is redirected if the sender cancels the 
	 * approval for a payment on paypal.com. Use the pay key to identify the payment as 
	 * follows: payKey=${payKey}.
	 * 
	 * @var unknown_type
	 */
	var $cancel_url;
	
	/**
	 * The currency of the payment. The default is USD.
	 * 
	 * @see https://merchant.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_api_nvp_currency_codes
	 * 
	 * @var string
	 */
	var $currency_code = 'USD';
	
	/**
	 * Recurring times. Number of times that subscription payments recur. 
	 * Specify an integer above 1. Valid only if you specify src="1".
	 * 
	 * @var boolean
	 */
	var $srt = 0;
	
	/**
	 * Recurring payments. Subscription payments recur unless subscribers 
	 * cancel their subscriptions before the end of the current billing cycle 
	 * or you limit the number of times that payments recur with the value that 
	 * you specify for srt.
	 * Allowable values are:
	 * 
	 * 		0 – subscription payments do not recur
	 * 		1 – subscription payments recur
	 * 		The default is 0.
	 * 
	 * @var boolean
	 */
	var $src = 1;
	
	/**
	 * 
	 * @var string
	 */
	var $notify_url;
	
	/**
	 * An identifier of the source that built the code for the button that the buyer 
	 * clicked, sometimes known as the build notation. Specify a value using the 
	 * following format:
	 * <Company>_<Service>_<Product>_<Country>
	 * 
	 * Substitute <Service> with an appropriate value from the following list:
	 * 		BuyNow
	 * 		AddToCart
	 * 		Donate
	 * 		Subscribe
	 * 		AutomaticBilling
	 * 		InstallmentPlan
	 * 		BuyGiftCertifcate
	 * 		ShoppingCart
	 * 
	 * Substitute <Product> with WPS always for Website Payments Standard payment 
	 * buttons and for the Website Payments Standard Cart Upload command.
	 * 
	 * Substitute <Country> with an appropriate two-letter country code from codes 
	 * defined by the ISO 3166-1 standard.
	 * 
	 * For example, a Buy Now button on your website that you coded yourself might 
	 * have the following line of code:  bn="DesignerFotos_BuyNow_WPS_US"
	 * 
	 * NOTE: HTML button code that you create on the PayPal website includes bn 
	 * variables with valid values generated by PayPal.
	 * 
	 * @var string
	 */
	var $bn = 'PP-SubscriptionsBF:btn_subscribeCC_LG.gif:NonHosted';
	
	/**
	 * Variables make up the first trial period
	 * 
	 * @var mixed
	 */
	var $a1;
	var $p1;
	var $t1;
	
	/**
	 * Variables make up the second trial period
	 * 
	 * @var mixed
	 */
	var $a2;
	var $p2;
	var $t2;
	
	/**
	 * Regular subscription price.
	 * 
	 * @var INT
	 */
	var $a3;
	
	/**
	 * Regular subscription duration. Required if you specify a1. Specify an 
	 * integer value in the allowable range for the units of duration that 
	 * you specify with t1.
	 * 
	 * @var INT
	 */
	var $p3;
	
	/**
	 * Regular subscription units of duration. Required if you specify a1.
	 * Allowable values are:
	 * 
	 * 		D – for days; allowable range for p2 is 1 to 90
	 * 		W – for weeks; allowable range for p2 is 1 to 52
	 * 		M – for months; allowable range for p2 is 1 to 24
	 * 		Y – for years; allowable range for p2 is 1 to 5
	 * 
	 * @var string
	 */
	var $t3;
	
	/**
	 * Variable is a passthru variable that comes back to us just as we sent it.
	 * 
	 * @var mixed
	 */
	var $custom;
	
	/**
	 * Variable determines wether or not this is a test purchase
	 * 
	 * @var string
	 */
	var $_test;
	
	/**
	 * The user can turn the trial offers on and off
	 * 
	 * @var string
	 */
	var $_trial_1;
	var $_trial_2;
	
	/**
	 * 
	 * 
	 * @example https://www.paypal.com/cgi-bin/webscr?cmd=_xclick-subscriptions
	 * 		&business=jonathonbyrd%40gmail%2ecom
	 * 		&lc=US
	 * 		&item_name=name&item_number=subid
	 * 		&no_note=1&no_shipping=
	 * 		&rm=1&return=https%3a%2f%2fwww%2emystore%2ecom%2fsuccess
	 * 		&cancel_return=https%3a%2f%2fwww%2emystore%2ecom%2fcancel
	 * 		&a1=29%2e00&p1=1&t1=M&a2=3%2e00&p2=1&t2=M&src=1&a3=49%2e00&p3=1&t3=M
	 * 		&currency_code=USD
	 * 		&srt=2
	 * 		&bn=PP%2dSubscriptionsBF%3abtn_subscribeCC_LG%2egif%3aNonHosted
	 * 		&notify_url=https%3a%2f%2fwww%2emywebsite%2ecom%2fPayPal_IPN
	 * 
	 */
	function toUrl()
	{
		$base = $this->_test ? 'https://www.sandbox.paypal.com/'
				: 'https://www.paypal.com/cgi-bin/webscr'; //'https://www.paypal.com/';
		
		return $base.'?'.$this->getQuerystring( 1 );
	}
	
	/**
	 * Function redirects the user to paypal immediately.
	 */
	function goNow()
	{
		header('Location: '.$this->toUrl());
		exit;
	}
	
	/**
	 * Turns the properties into a querystring
	 * 
	 * @return string
	 */
	function getQuerystring( $urlencode = false )
	{
		$query = array();
		foreach($this->getProperties() as $property => $value)
		{
			$method = 'get'.str_replace(' ','',ucwords(str_replace('_',' ',$property)));
			if (method_exists($this, $method)) {
				$value = $this->$method( $value );
			}
			
			if ($value !== false)
			{
				if ($urlencode) {
					$query[] = $property.'='.urlencode($value);
				}
				else {
					$query[] = $property.'='.$value;
				}
			}
		}
		return implode('&', $query);
	}
	
	/**
	 * Returns the paypal email address
	 */
	function getBusiness( $value = NULL )
	{
		return apply_filters('ms_business', $value ?$value :$this->business, $this);
	}
	
	/**
	 * Returns the language of WordPress
	 */
	function getLc( $value = NULL )
	{
		return strtoupper( $value ?$value :$this->lc );
	}
	
	/**
	 * Whether or not payments are recurring. This return value is completely
	 * dependent on the srt value
	 * 
	 * 0 – subscription payments do not recur
	 * 1 – subscription payments recur
	 */
	function getSrc( $value = NULL )
	{
		return $this->src;
	}
	
	/**
	 * Returns the currency code
	 */
	function getCurrencyCode( $value = NULL )
	{
		return $value ?$value : $this->currency_code;
	}
	
	/**
	 * Returns the IPN url
	 */
	function getNotifyUrl( $value = NULL )
	{
		return apply_filters('ms_notify_url', $value ?$value: $this->notify_url, $this);
	}
	
	/**
	 * Returns the receipt link
	 */
	function getReturnUrl( $value = NULL )
	{
		return apply_filters('ms_return_url', $value ?$value: $this->return_url, $this);
	}
	
	/**
	 * Returns the cancellation link
	 */
	function getCancelUrl( $value = NULL )
	{
		return apply_filters('ms_cancel_url', $value ?$value: $this->cancel_url, $this);
	}
	
	/**
	 * 
	 */
	function getA1( $value = NULL )
	{
		if (!$this->_trial_1) return false;
		$a = (int)trim(str_replace('$','',$this->a1));
		return $value ?$value : ($a?$a:0);
	}
	
	/**
	 * 
	 */
	function getP1( $value = NULL )
	{
		if (!$this->_trial_1) return false;
		return $value ?$value : $this->p1;
	}
	
	/**
	 * 
	 */
	function getT1( $value = NULL )
	{
		if (!$this->_trial_1) return false;
		return $value ?$value : $this->t1;
	}
	
	/**
	 * 
	 */
	function getA2( $value = NULL )
	{
		if (!$this->_trial_2) return false;
		$a = (int)trim(str_replace('$','',$this->a2));
		return $value ?$value : ($a?$a:0);
	}
	
	/**
	 * 
	 */
	function getP2( $value = NULL )
	{
		if (!$this->_trial_2) return false;
		return $value ?$value : $this->p2;
	}
	
	/**
	 * 
	 */
	function getT2( $value = NULL )
	{
		if (!$this->_trial_2) return false;
		return $value ?$value : $this->t2;
	}
	
	/**
	 * 
	 * @param unknown_type $value
	 * @return Ambigous <unknown, number>
	 */
	function getA3( $value = NULL )
	{
		$a = (int)trim(str_replace('$','',$this->a3));
		return $value ?$value : ($a?$a:0);
	}
	
	/**
	 * 
	 */
	function getFirstPayment()
	{
		if ($this->_trial_1) {
			return $this->getA1();
		}
		elseif ($this->_trial_2) {
			return $this->getA2();
		}
		else{
			return $this->getA3();
		}
		return false;
	}
	
	
	function debug()
	{
		$query = array();
		foreach($this->getProperties() as $property => $value)
		{
			$method = 'get'.str_replace(' ','',ucwords(str_replace('_',' ',$property)));
			if (method_exists($this, $method)) {
				$value = $this->$method( $value );
			}
			
			if ($value)
				$query[$property] = $value;
		}
		
		?><pre><?php var_export($query); ?></pre><?php 
	}
	
	/**
	 * Constructor.
	 * 
	 */
	function __construct( $options = array() )
	{
		//initializing
		$this->bind($options);
	}
	
	/**
	 * Method to bind an associative array or object to the JTable instance.This
	 * method only binds properties that are publicly accessible and optionally
	 * takes an array of properties to ignore when binding.
	 *
	 * @param   mixed  $src     An associative array or object to bind to the JTable instance.
	 * @param   mixed  $ignore  An optional array or space separated list of properties to ignore while binding.
	 *
	 * @return  boolean  True on success.
	 *
	 * @link    http://docs.joomla.org/JTable/bind
	 * @since   11.
	 */
	public function bind($src, $ignore = array())
	{
		// If the source value is not an array or object return false.
		if (!is_object($src) && !is_array($src))
		{
			trigger_error('Bind failed as the provided source is not an array.');
			return false;
		}

		// If the source value is an object, get its accessible properties.
		if (is_object($src))
		{
			$src = get_object_vars($src);
		}

		// If the ignore value is a string, explode it over spaces.
		if (!is_array($ignore))
		{
			$ignore = explode(' ', $ignore);
		}

		// Bind the source value, excluding the ignored fields.
		foreach ($this->getProperties( false ) as $k => $v)
		{
			// Only process fields not in the ignore array.
			if (!in_array($k, $ignore))
			{
				if (isset($src[$k]))
				{
					$this->$k = $src[$k];
				}
			}
		}

		return true;
	}
	
	/**
	 * Set the object properties based on a named array/hash.
	 *
	 * @param   mixed  $properties  Either an associative array or another object.
	 *
	 * @return  boolean
	 *
	 * @since   11.
	 *
	 * @see     set() 
	 */
	public function setProperties($properties)
	{
		if (is_array($properties) || is_object($properties))
		{
			foreach ((array) $properties as $k => $v)
			{
				// Use the set function which might be overridden.
				$this->set($k, $v);
			}
			return true;
		}

		return false;
	}
	
	/**
	 * Modifies a property of the object, creating it if it does not already exist.
	 *
	 * @param   string  $property  The name of the property.
	 * @param   mixed   $value     The value of the property to set.
	 *
	 * @return  mixed  Previous value of the property.
	 *
	 * @since   11.
	 */
	public function set($property, $value = null)
	{
		$previous = isset($this->$property) ? $this->$property : null;
		$this->$property = $value;
		return $previous;
	}
	
	/**
	 * Returns an associative array of object properties.
	 *
	 * @param   boolean  $public  If true, returns only the public properties.
	 *
	 * @return  array 
	 *
	 * @see     get()
	 */
	public function getProperties($public = true)
	{
		$vars = get_object_vars($this);
		if ($public)
		{
			foreach ($vars as $key => $value)
			{
				if ('_' == substr($key, 0, 1))
				{
					unset($vars[$key]);
				}
			}
		}

		return $vars;
	}
	
	/**
	 * 
	 * contains the current instance of this class
	 * @var object
	 */
	static $_instances = null;
	
	/**
	 * Method is called when we need to instantiate this class
	 * 
	 * @param array $options
	 */
	public static function getInstance( $id, $options = array() )
	{
		if (!isset(self::$_instances[$id]))
		{
			$class = get_class();
			$options['_id'] = $id;
			self::$_instances[$id] =& new $class($options);
		}
		return self::$_instances[$id];
	}
}