Why am I getting gibberish characters?

I'm making a PHP script to reverse the text within an HTML document to handle badly converted Hebrew PDFs. (sigh :))

Everything works, however the script has a very strange output. Only SOME of the characters, instead of staying Hebrew letters, turn into blank characters (those black diamonds with question marks).

I tried some solution I could find on SO and beyond but nothing changed. Perhaps you can enlighten me?

You can check the script in action here: pilau.phpnet.us/html_invert.php, and this is the entire source code:

<!DOCTYPE html>
<html lang="he-IL">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>
<body>
    <form action="html_invert.php" method="post" enctype="application/x-www-form-urlencoded">
        <textarea id="html_code" name="html_code" rows="30" cols="80"><?php
            if (isset($_POST['html_code']))
            {
                function invert_string ($str) {
                        $new_str = '';
                        $i = strlen($str);
                        while ($i > 0) {
                            $new_str .= substr($str, --$i, 1);
                        }
                        return '>'.$new_str.'<';
                    }

                    echo htmlspecialchars(preg_replace('/>(\s*.*\s*)</imUue', 'invert_string("$1")', stripslashes($_POST['html_code'])));
            }
            else { echo 'paste your text here'; }
        ?></textarea>
        <br />
        <input type="submit" value="Process HTML" />
    </form>
</body>
</html>

Solution

I wanted to mark this question as answered, so here is the solution, courtesy of Wooble and Matthew as described in the comments on the question above:

I used mb_substr('UTF-8') and mb_strlen('UTF-8'), as well as replaced stripslashes() with this regular expression: preg_replace(array('/\x5C(?!\x5C)/u', '/\x5C\x5C/u'), array('','\\'), $_POST['html_code']).

Thus the complete code is as follows:

    <textarea id="html_code" name="html_code" rows="30" cols="80"><?php
        if (isset($_POST['html_code']))
        {
            function add_delimiters ($str, $deli, $optional_suffix) {
                return (isset($optional_suffix) ? $deli.$str.$optional_suffix : $deli.$str.$deli);
            }

            function reverse_string ($str) {
                $new_str = '';
                $i = mb_strlen($str, 'UTF-8');
                while ($i > 0) {
                    $new_str .= mb_substr($str, --$i, 1, 'UTF-8');
                }
                return $new_str;
            }

            function utf_stripslashes ($str) {
                return preg_replace(array('/\x5C(?!\x5C)/u', '/\x5C\x5C/u'), array('','\\'), $str);
            }

            function strip_blank_lines ($str) {
                return preg_replace("/(^[\r\n]*|[\r\n]+)[\s\t]*[\r\n]+/u", "\n", $str);
            }

            function reverse_html_content ($html) {
                return preg_replace('/>(\s*.*\s*)</imUue', 'add_delimiters(reverse_string("$1"), ">", "<")', utf_stripslashes($html));
            }

            function clear_unsupported_css ($style) {
                return preg_replace(array('/top:\s{0,1}([0-9]*(?!px));{0,1}/iu', '/left:\s{0,1}([0-9]*(?!px));{0,1}/iu'), array('top:$1px;', 'left:$1px;'), $style);
            }

            function process_inline_style ($html, $func) {
                return preg_replace('/style="[a-zA-Z0-9:;\s{0,1}]*"/imUue', $func.'("$0")', $html);
            }               

            echo strip_blank_lines(htmlspecialchars(process_inline_style(reverse_html_content($_POST['html_code']), 'clear_unsupported_css')));
        }
        else { echo 'paste your text here'; }
    ?></textarea>