example.php

<?php
// First, let's just build a string of content.

// Method 1: If you using single quotes and you want to insert
// variables, you MUST open and close the string, and append chunks
// together with periods.

$str = '[lightbox link="' . $videourl . '" thumb="' . $videothumb . '"]';

// Method 2: When wrapping the string in double quotes, you can actually
// insert variables directly into the string. But you need to make
// sure and escape your double quotes within with a backslash.

$str = "[lightbox link=\"$videourl\" thumb=\"$videothumb\"]";

// Method 3: Both options above can look a little messy when you look
// back at them later. This why I really like to use PHP's sprintf()
// function, to make the code look a bit cleaner and give a more clear
// visual of what's happenning.

$str = sprintf(
    '[lightbox link="%s" thumb="%s"]',
    $videourl,
    $videothumb
);

// And then just pass your string to the do_shortcode() function
// and echo it out.

echo do_shortcode( $str );

// And of course, you could build the string with any of the methods
// above directly within the do_shortcode() call, too.

echo do_shortcode( '[lightbox link="' . $videourl . '" thumb="' . $videothumb . '"]' );