Keep in mind that you should never trust user input - particularly for "mixed-bag" input containing a combination of plain text and markup or scripting code.
Well, consider someone sending '&<script>alert('XSS');</script>' to your PHP script:
$var = "&<script>alert('XSS');</script>";
$var = (htmlspecialchars_decode($var) == $var) ? htmlspecialchars($var) : $var;
Since '&' decodes into '&', (htmlspecialchars_decode($var) == $var) will be -false-, thus returning $var without that it's escaped. In consequence, the script-tags are untouched, and you've just opened yourself to XSS.
There is, unfortunately, no reliable way to determine whether HTML is escaped or not that does not come with this caveat that I know of. Rather than try and catch the case 'I've already encoded this', you are better off avoiding double-escaping by simply escaping the HTML as close to the actual output as you can muster, e.g. in the view in an MVC development structure.