Auto-play YouTube embeds

gistfile1.php

<?php

add_filter( 'embed_oembed_html', 'blogsuccessjrnl_youtube_autoplay', 10, 4 );

function blogsuccessjrnl_youtube_autoplay( $html, $url, $attr, $post_ID ) {

	// Abort if not in a single-post view. You don't want these auto-playing on you homepage.
	if ( ! is_singular() )
		return $html;

	// Abort if not YouTube
	if ( ! in_array( parse_url( $url, PHP_URL_HOST ), array( 'youtube.com', 'www.youtube.com' ) ) )
		return $html;

	// Add &autoplay=1 onto the end of the URL
	$html = preg_replace( '#src="([^"]+)"#', 'src="$1&autoplay=1"', $html );

	// This is a filter, so you MUST return the variable
	return $html;
}