Best way to escape characters in URL?

The & (ampersand) character, for one, is enough to invalidate a stock ZenCart site.

I looked at some old threads and didn't find exactly what I was looking for in an effort to validate all of my URLs. One article did reference the /includes/functions/html_output.php file.

At line 57, I changed the line from this:

$separator = '&';

To this:

$separator = '&';

Which seems to solve the problem.

Can any of the proficient PHP programmers out there confirm if this is the best location to make this modification? Am I just lucky that it helped my particular case, or is there a chance URLs might invalidate with other characters that are not escaped?

You'll notice that line 94 already does that for you:

    $link = ereg_replace('&', '&', $link);

If that's the case, why were all of my ampersands unescaped? That is, & instead of & ?

All of the links or only certain links? The URL?