Output WordPress content with a class on the first para

first_para_class.php

<?php

/**
 * Output content with a class on the first para
 *
 * @param	string	$class	Default: 'intro'
 * @return	void
 */
function pilau_the_content_first_para_class( $class = 'intro' ) {
	$content = apply_filters( 'the_content', get_the_content() );
	echo preg_replace( '/<p([^>]+)?>/', '<p$1 class="' . $class . '">', $content, 1 );
}