How to substitute http links if the user did not set it?

Question

PHP

Users forget to copy the link completely
And it turns out in the code that the links work on the site, as internal how to substitute http links if the user inserted the link without it
<a href="youtube.com">youtube.com</a>
<a href="http://youtube.com">youtube.com</a>

Answer 1

The medium editor has linkValidation: true which solves all problems

Answer 2

No way. Learn how to link properly.
Face the problem that the desired internal link is replaced with an external one

Answer 3

if(substr($url, 0,3) != “http”) {
$url = “ http://“ . $url;
}
Something like this, well, you can also check on https