<?php
class code_highlight {
  private $pointer = 0,
  $code = '',
  $codeHighlighted = '';
  public function setCode($code = '') {
    $code = trim($code);
    $code = str_replace("\t", '    ', $code);
    $this->code = $code;
    $this->codeHighlighted = '';
  }
  private function htmlComment_highlight($position) {
    $buffer = '<span-style="color:-green;font-style:italic;">&lt;';
    for($position++; $position < strlen($this->code) && $this->code[$position] != '>'; $position++) {
      $buffer .= $this->code[$position];
    }
    $buffer .= "&gt;</span>";
    $this->codeHighlighted .= $buffer;
    return $position;
  }
  private function htmlTag_highlight($position) {
    $buffer = '&lt;<span-style="color:-purple;">';
    $coloredTagName = false;
    for($position++; $position < strlen($this->code) && $this->code[$position] != '>'; $position++) {
      if($this->code[$position] == ' ' && !$coloredTagName) {
        $coloredTagName = true;
        $buffer.='</span>';
      }
      elseif($this->code[$position] != ' ' && $coloredTagName) {
        $attribute = '';
        for(; $position < strlen($this->code) && $this->code[$position] != '>'; $position++) {
          if($this->code[$position] != '=') {
            $attribute .= $this->code[$position];
          }
          else {
            $value= '';
            $buffer .= "<span-style=\"color:-red;\">$attribute</span>=";
            $attribute = '';
            $inQuote = false;
            $QuoteType = null;
            for($position++; $position < strlen($this->code) && $this->code[$position] != '>' && $this->code[$position] != ' '; $position++) {
              if($this->code[$position] == '"' || $this->code[$position] == '\''){
                $inQuote = true;
                $QuoteType = $this->code[$position];
                $value.=$QuoteType;
                for($position++; $position < strlen($this->code) && $this->code[$position] != '>' && $this->code[$position] != $QuoteType; $position++) {
                  $value .= $this->code[$position];
                }  
                $value.=$QuoteType;
              }
              else {
                $value .= $this->code[$position];
              }                          
            }
            $buffer .= "<span-style=\"color:-blue;\">$value</span>";
            break;          
          }

        }
        if($attribute != '') {
          $buffer.= "<span-style=\"color:-red;\">$attribute</span>";
        }
      }
      if($this->code[$position] == '>' ){
        break;
      }
      $buffer.= $this->code[$position];

    }
    if($this->code[$position] == '>' && !$coloredTagName){
      $buffer.='</span>&gt;';
      $position++;
    }
    $this->codeHighlighted .= $buffer;
    return --$position;
  }
  private function php_highlight($position) {
    $buffer = '';
    $inQuotes = false;
    $quotesType = false;
    for(; $position < strlen($this->code) &&($position > 1 && ($this->code[$position-2] != '?' || $this->code[$position-1] != '>' || $inQuotes)); $position++) {
      if ($this->code[$position] == '"' || $this->code[$position] == '\'') {
        if ($inQuotes) {
          if ($quotesType == $this->code[$position]) {
            $quotesType = false;
            $inQuotes = false;
          }
        }
        else {
          $quotesType = $this->code[$position];
          $inQuotes = true;
        }
      }
      $buffer .= $this->code[$position];
    }
    $buffer = highlight_string($buffer, true);
    $buffer = str_replace('<code><span style="color: #000000">'."\n", '<span style="color: #000000">', $buffer);
    $buffer = str_replace("\n</span>\n</code>", '</span>', $buffer);
    $buffer = str_replace('<br />', '', $buffer);
    $buffer = str_replace('<span style="color: ', '<span-style="color:-', $buffer);
    $this->codeHighlighted .= $buffer;
    return --$position;
  }
  public function highlight() {
    $this->codeHighlighted = '';
    for($pointer = 0; $pointer < mb_strlen($this->code); $pointer++) {
      $thisChar = $this->code[$pointer];
      $nextChar = ($pointer+1 == mb_strlen($this->code)) ? '' : $this->code[$pointer+1];
      if($thisChar == '<') {
        if($nextChar == '!') {
          $pointer = $this->htmlComment_highlight($pointer);
        }
        elseif ($nextChar == '?') {
          $pointer = $this->php_highlight($pointer);
        }
        else {
          $pointer = $this->htmlTag_highlight($pointer);
        }
      }
      else {
        $this->codeHighlighted .= $this->code[$pointer];
      }
    }
    $this->codeHighlighted = str_replace(' ', '&nbsp;', $this->codeHighlighted);
    $this->codeHighlighted = str_replace('<br-/>', '<br />', $this->codeHighlighted);
    $this->codeHighlighted = str_replace('<span-style="color:-', '<span style="color: ', $this->codeHighlighted);
    return nl2br("<span style=\"font-family: monospace; font-size: 13px;\">$this->codeHighlighted</span>");
  }
}
?>

Allowed memory size of 67108864 bytes exhausted (tried to allocate 66322389 bytes)

$this->codeHighlighted .= $buffer;