initial plugin

.gitmodules

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+[submodule "includes/twitter-cards-php"]
+	path = includes/twitter-cards-php
+	url = https://github.com/niallkennedy/twitter-cards-php.git

class-twitter-card-wp.php

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+<?php
+
+if ( ! class_exists( 'Twitter_Card' ) )
+	require_once( dirname(__FILE__) . '/includes/twitter-cards-php/twitter-card.php' );
+
+class Twitter_Card_WP extends Twitter_Card {
+
+	/**
+	 * Test if given URL is valid and matches allowed schemes
+	 *
+	 * @since 1.0
+	 * @param string $url URL to test
+	 * @param array $allowed_schemes one or both of http, https
+	 * @return bool true if URL can be parsed and scheme allowed, else false
+	 */
+	public static function is_valid_url( $url, $allowed_schemes = null ) {
+		if ( parent::is_valid_url( $url, $allowed_schemes ) && esc_url_raw( $url, $allowed_schemes ) )
+			return true;
+		return false;
+	}
+
+	/**
+	 * Build a single <meta> element from a name and value
+	 *
+	 * @param string $name name attribute value
+	 * @param string|int $value value attribute value
+	 * @param bool $xml include a trailing slash for XML. encode attributes for XHTML in PHP 5.4+
+	 * @return meta element or empty string if name or value not valid
+	 */
+	public static function build_meta_element( $name, $value, $xml = false ) {
+		if ( ! ( is_string( $name ) && $name && ( is_string( $value ) || ( is_int( $value ) && $value > 0 ) ) ) )
+			return '';
+		return '<meta name="' . esc_attr( self::PREFIX . ':' . $name ) . '" value="' . esc_attr( $value ) . '"' . ( $xml === true ? ' />' : '>' ) . "\n";
+	}
+
+	/**
+	 * Pass all URLs through esc_url_raw. Unset the property if URL rejected
+	 *
+	 * @since 1.0
+	 * @uses esc_url_raw()
+	 */
+	private function filter_urls() {
+		if ( isset( $this->url ) ) {
+			$this->url = esc_url_raw( $this->url );
+			if ( ! $this->url )
+				unset( $this->url );
+		}
+
+		if ( isset( $this->image ) && isset( $this->image->url ) ) {
+			$this->image->url = esc_url_raw( $this->image->url );
+			if ( ! $this->image->url )
+				unset( $this->image );
+		}
+
+		if ( isset( $this->video ) && isset( $this->video->url ) ) {
+			$this->video->url = esc_url_raw( $this->video->url );
+			if ( $this->video->url ) {
+				if ( isset( $this->video->stream ) && isset( $this->video->stream->url ) )
+					$this->video->stream->url = esc_url_raw( $this->video->stream->url );
+					if ( ! $this->video->stream->url )
+						unset( $this->video->stream );
+			} else {
+				unset( $this->video );
+			}
+		}
+	}
+
+	/**
+	 * Build a string of <meta> elements representing the object
+	 * Pass URLs through esc_url_raw to preserve site preferences
+	 *
+	 * @since 1.0
+	 * @param string $style markup style. "xml" adds a trailing slash to the meta void element
+	 * @return string <meta> elements or empty string if minimum requirements not met
+	 */
+	private function generate_markup( $style = 'html' ) {
+		$xml = false;
+		if ( $style === 'xml' )
+			$xml = true;
+		$this->filter_urls();
+		$t = apply_filters( 'twitter_cards_properties', $this->toArray() );
+		if ( ! is_array( $t ) || empty( $t ) )
+			return '';
+		$s = '';
+		foreach ( $t as $name => $value ) {
+			$s .= self::build_meta_element( $name, $value, $xml );
+		}
+		return $s;
+	}
+
+	/**
+	 * Output object properties as HTML meta elements with name and value attributes
+	 *
+	 * @return string HTML <meta> elements or empty string if minimum requirements not met for card type
+	 */
+	public function asHTML() {
+		return $this->generate_markup();
+	}
+
+	/**
+	 * Output object properties as XML meta elements with name and value attributes
+	 *
+	 * @since 1.0
+	 * @return string XML <meta> elements or empty string if minimum requirements not met for card type
+	 */
+	public function asXML() {
+		return $this->generate_markup( 'xml' );
+	}
+}
+?>

twitter-cards.php

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+<?php
+/**
+ * Plugin Name: Twitter Cards
+ * Plugin URI: https://github.com/niallkennedy/twitter-cards
+ * Description: Add Twitter Cards markup to individual posts.
+ * Author: Niall Kennedy
+ * Author URI: http://www.niallkennedy.com/
+ * Version: 1.0
+ */
+
+if ( ! class_exists( 'Twitter_Cards' ) ):
+/**
+ * Add Twitter Card markup to document <head>
+ *
+ * @since 1.0
+ * @version 1.0
+ */
+class Twitter_Cards {
+	/**
+	 * Attach Twitter cards markup to wp_head if single post view
+	 *
+	 * @since 1.0
+	 */
+	public static function init() {
+		if ( is_single() )
+			add_action( 'wp_head', 'Twitter_Cards::markup' );
+	}
+
+	/**
+	 * Build a Twitter Card object. Possibly output markup
+	 *
+	 * @since 1.0
+	 */
+	public static function markup() {
+		global $post;
+
+		if ( ! class_exists( 'Twitter_Card_WP' ) )
+			require_once( dirname(__FILE__) . '/class-twitter-card-wp.php' );
+
+		$card = new Twitter_Card_WP();
+		$card->setURL( apply_filters( 'rel_canonical', get_permalink() ) );
+		$post_type = get_post_type();
+		if ( post_type_supports( $post_type, 'title' ) )
+			$card->setTitle( get_the_title() );
+		if ( post_type_supports( $post_type, 'excerpt' ) ) {
+			// one line, no HTML
+			$card->setDescription( self::clean_description( apply_filters( 'the_excerpt', get_the_excerpt() ) ) );
+		}
+		// does current post type and the current theme support post thumbnails?
+		if ( post_type_supports( $post_type, 'thumbnail' ) && function_exists( 'has_post_thumbnail' ) && has_post_thumbnail() ) {
+			list( $post_thumbnail_url, $post_thumbnail_width, $post_thumbnail_height ) = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'full' );
+			$card->setImage( $post_thumbnail_url, $post_thumbnail_width, $post_thumbnail_height );
+		}
+
+		if ( apply_filters( 'twitter_cards_htmlxml', 'html' ) === 'xml' )
+			echo $card->asXML();
+		else
+			echo $card->asHTML();
+	}
+
+	public static function clean_description( $description ) {
+		if ( ! ( is_string( $description ) && $description ) )
+			return '';
+
+		$description = wp_strip_all_tags( strip_shortcodes( $description ) );
+		$description = trim( str_replace( array( "\r\n", "\r", "\n" ), ' ', $description ) );
+		$excerpt_more = trim( wp_strip_all_tags( apply_filters('excerpt_more', '[...]') ) );
+		if ( $excerpt_more ) {
+			$excerpt_more_length = strlen( $excerpt_more );
+			if ( strlen( $description ) > $excerpt_more_length && substr_compare( $description, $excerpt_more, $excerpt_more_length * -1, $excerpt_more_length ) === 0 ) {
+				$description = trim( substr( $description, 0, $excerpt_more_length * -1 ) );
+			}
+		}
+		return $description;
+	}
+}
+add_action( 'wp', 'Twitter_Cards::init' );
+endif;
+?>