Get the thumbnail image of Youtube by Youtube Video URL.

get_youtube_thumbnail.php

function get_youtube_thumb($video_url)
{
	
	if(preg_match('/src="([^"]+)"/', $video_url, $match))
		$video_url = $match[1];
	
	$ytRegExp = "/^(?:https?:\/\/)?(?:www\.)?(?:youtu\.be\/|youtube\.com\/(?:embed\/|v\/|watch\?v=|watch\?.+&v=))((\w|-){11})(?:\S+)?$/";
		                       	preg_match($ytRegExp, $video_url, $ytMatch);
	$large = "";	                       	
	if ($ytMatch && strlen($ytMatch[1]) === 11) 
	{   
		$youtubeVideoId = $thumbURL = "";
		$youtubeVideoId = $ytMatch[1];

		if($youtubeVideoId)
			$thumbURL = 'https://img.youtube.com/vi/'.$youtubeVideoId.'/0.jpg';
		else
			$thumbURL = get_template_directory_uri() . '/assets/images/video-placeholder.png';
	}
	$large = $thumbURL;
	return $large;
}