bbcode.php

<?php

function getEmbed($value)
{
  $video = $value[1];	
  $output = explode( '?v=', $video);
  $player = '<iframe src="https://www.youtube.com/embed/%s"></iframe>';	
  return sprintf($player, $output[1]);
}


function bbcode($content)
{
	$pattern = '@\[video\](.*?)\[\/video\]@';
	return preg_replace_callback($pattern, "getEmbed", $content);
}

$string = '[video]https://www.youtube.com/watch?v=YgHRBEYy6mM[/video]';
echo bbcode($string);