使用简单的PHP的Html自定义标记符号

Question

Fiddle

INPUT - Well, **Mr. Anderson** is a *_tall_* guy.

输出 - Well, <strong>Mr. Anderson</strong> is a <em>_tall_</em> guy.

预期输出 - Well, <strong>Mr. Anderson</strong> is a <em><span class="underlined">tall</span></em> guy.

我能够创建一个符号标记系统，但是当符号组合时它不能完成工作。 有人可以帮我吗？ 这与stackoverflow＆＃39; s QA Markup system非常相似 有没有人有更好的方法。我不想使用插件

 $str = "Well, **Mr. Anderson** is a *_tall_* guy.";
 $str_ar = explode(" ",$str);
 $new_str = "";
    foreach($str_ar as $s){
        $new_str .= doreplace($s);
    }
    echo '<br />'. $new_str;

    function doreplace($str){
        $new_str = "";
        $new_str .= doCheckFB($str);
            //$new_str .= checkFB($str,'back');
        $new_str = " ".$new_str." ";
        return $new_str;
    }
    function doCheckFB($str){
        $new_str = checkFB($str,'front');
        $new_str = checkFB($new_str,'back');
        return $new_str;
    }
    function checkFB($str,$pos){
            $new_str ="";
            if($pos == 'front'){
                $switchCase = substr($str,0,1);     
            }else if($pos == 'back'){
                $switchCase = substr($str,strlen($str) - 1,1);      
            }

            switch($switchCase){
            case '*':
                    if($pos == 'front'){
                        $subSwitch = substr($str,1,1);
                    }else if($pos == 'back'){
                        $subSwitch = substr($str,strlen($str) - 2,1);
                    }
                if($subSwitch == '*'){
                    $globalAssoc = "**";
                    $notationF = "<strong>";
                    $notationB = "</strong>";
                    $length = 2;

                }else{
                    $globalAssoc = "*";
                    $notationF = "<em>";
                    $notationB = "</em>";
                    $length = 1;
                }
            break;
            case '_':
                    $globalAssoc = "*";
                    $notationF = "<span class=\"underlined\">";
                    $notationB = "</span>";
                    $length = 1;                
            break;
            default :
                $new_str .= $str;
                return $new_str;
        }
        if($pos=='front'){
            $new_str .= getReplacedDone($str,$globalAssoc,$notationF,$length,"front");
        }else if($pos == 'back'){   
            $new_str .= getReplacedDone($str,$globalAssoc,$notationB,$length,"back");
        }
        return $new_str;
    }
    function getReplacedDone($str,$globalAssoc,$notation,$length,$pos){
                     $len = strlen($str);
                     $new_str = "";
                     if($pos=='front'){
                        $tmp_str = substr($str,$length);
                        $new_str .= $notation. $tmp_str;
                        $new_str = checkFB($new_str,'back');                        
                    }else if($pos == 'back'){
                        $tmp_str = substr($str,0,$len - $length);
                        $new_str .= $tmp_str.$notation;
                    }       
                    return $new_str;
    }