Adding Twitter cards to a WordPress website. Add this to your header.php

twitter.php

<?php 
  	if(is_singular()) { ?>
		<meta name="twitter:card" content="summary">
		<meta name="twitter:url" content="<?php echo get_permalink($post->ID); ?>">
		<meta name="twitter:title" content="<?php echo $post->post_title;?>">
		<meta name="twitter:description" content="<?php echo substr( $post->post_content, 0, 150 );?>">
		<?php
      			// you will need to hardcode this or use a plugin that allows twitter info to be added to authors
			$twitter = the_author_meta('twitter', $post->post_author); 
			if($twitter){
		    		echo "<meta name='twitter:creator' content='$twitter'>";
		  	} 
    			$url = wp_get_attachment_url( get_post_thumbnail_id($post->ID) ); 
			if($url){
		    		echo "<meta name='twitter:image' content='$url'>";
			}
    		?>
	}
?>