A useful PHP function that uses regex to get the YouTube video ID from however your user decided to copy the YouTube Link!

youtube_id_from_url

<?php
	/**
	 * get youtube video ID from URL
	 *
	 * @param string $url
	 * @return string Youtube video id or FALSE if none found. 
	 */
	function youtube_id_from_url($url) {
	    $pattern = 
	        '%^# Match any youtube URL
	        (?:https?://)?  # Optional scheme. Either http or https
	        (?:www\.)?      # Optional www subdomain
	        (?:             # Group host alternatives
	          youtu\.be/    # Either youtu.be,
	        | youtube\.com  # or youtube.com
	          (?:           # Group path alternatives
	            /embed/     # Either /embed/
	          | /v/         # or /v/
	          | /watch\?v=  # or /watch\?v=
	          )             # End path alternatives.
	        )               # End host alternatives.
	        ([\w-]{10,12})  # Allow 10-12 for 11 char youtube id.
	        $%x'
	        ;
	    $result = preg_match($pattern, $url, $matches);
	    if (false !== $result) {
	        return $matches[1];
	    }
	    return false;
	}