plugin.php

<?php
/**
 * Campaign Monitor Shortcode & Reversal
 */

/**
 * Embed Reversal
 *
 * Looks form a <form> that looks like a Campaign Monitor subscribe
 * form during pre_kses, and converts it into a shortcode if possible.
 *
 * @see wcorg_campaign_monitor_shortcode
 */
function wcorg_campaign_monitor_embed_to_shortcode( $content ) {
	/*
	<form action="http://username.createsend.com/t/t/s/sjlk/" method="post" id="subForm">
	<div>
	<label for="name">Name:</label><br /><input type="text" name="cm-name" id="name" /><br />
	<label for="sjlk-sjlk">Email:</label><br /><input type="text" name="cm-sjlk-sjlk" id="sjlk-sjlk" /><br />
	<input type="submit" value="Subscribe" />
	</div>
	</form>
	*/
	if ( false === stripos( $content, '<form' ) && false === stripos( $content, '.createsend.com' ) )
		return $content;

	$matches = array();
	$regexp = '#<form(.+?)action=("|\')(https?://[^\.]+\.createsend\.com/[^/]+/[^/]+/[^/]+/[^/]+/)\2([^<]|[<](?!/form))+</form>#is';
	if ( ! preg_match_all( $regexp, $content, $matches ) || ! isset( $matches[3] ) )
		return $content;

	// Loop through found forms and replace them with shortcodes.
	foreach ( $matches[3] as $key => $url ) {
		$shortcode = sprintf( '[campaign-monitor url="%s"]', esc_url( $url ) );
		$content = str_replace( $matches[0][ $key ], $shortcode, $content );
	}

	return $content;
}
add_filter( 'pre_kses', 'wcorg_campaign_monitor_embed_to_shortcode' );

/**
 * Shortcode Callback
 *
 * Results in a form with the appropriate attributes and fields.
 * The only required attribute is the URL, which is the action URL
 * in a Campaign Monitor form.
 *
 * Example: [campaign-monitor url="http://username.createsend.com/t/t/s/sjlk/"]
 */
function wcorg_campaign_monitor_shortcode( $attr ) {
	$attr = shortcode_atts( array(
		'url'=> '',
		'subscribe_label' => 'Subscribe',
	), $attr );

	$attr['url'] = trailingslashit( esc_url_raw( $attr['url'] ) );
	$parsed_url = parse_url( $attr['url'] );

	if ( empty( $parsed_url['host'] ) || ! preg_match( '#\.createsend\.com$#i', $parsed_url['host'] ) )
		return 'Invalid Campaign Monitor URL.';

	$matches = array();
	if ( ! preg_match( '#^/[^/]+/[^/]+/[^/]+/([^/]+)/$#i', $parsed_url['path'], $matches ) )
		return 'Invalid Campaign Monitor URL.';

	$list_id = $matches[1];
	$email_name_attr = sprintf( 'cm-%s-%s', $list_id, $list_id );
	ob_start();
	?>
	<form class="wcorg-campaign-monitor" action="<?php echo esc_url( $attr['url'] ); ?>" method="post">
		<label for="wcorg-cm-name">Name:</label>
		<input type="text" name="cm-name" id="wcorg-cm-name" />
		<label for="wcorg-cm-email">Email:</label>
		<input type="text" name="<?php echo esc_attr( $email_name_attr ); ?>" id="wcorg-cm-email" />
		<input type="submit" value="<?php echo esc_attr( $attr['subscribe_label'] ); ?>" />
	</form>
	<?php
	$contents = ob_get_contents();
	ob_end_clean();
	return $contents;
}
add_shortcode( 'campaign-monitor', 'wcorg_campaign_monitor_shortcode' );