Html custom markup notations using simple php -

fiddle

input - well, **mr. anderson** *_tall_* guy.

output - well, <strong>mr. anderson</strong> <em>_tall_</em> guy.

expected output - well, <strong>mr. anderson</strong> <em><span class="underlined">tall</span></em> guy.

i able make notation markup system not doing job when notations combined . can me please ? similar stackoverflow's qa markup system has better approach. , don't use plugins

 $str = "well, **mr. anderson** *_tall_* guy.";  $str_ar = explode(" ",$str);  $new_str = "";     foreach($str_ar $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;     }