<?php
function convertSpecialToHTML($char) {
    $special = ['#', '&', '+', '\\'];

    if (in_array($char, $special)) {
        return '&#'.ord($char).';';
    } else {
        return $char;
    }
}

echo convertSpecialToHTML('#');  //&#35;
echo convertSpecialToHTML("\\"); //&#92;
echo convertSpecialToHTML("&");  //&#38;
echo convertSpecialToHTML("+");  //&#43;