Memory optimization

<?php
require('fpdf.php');

class FPDF2File extends FPDF
{
protected $f;

public function Open($file='doc.pdf')
{
    $this->f = fopen($file,'wb');
    if(!$this->f)
        $this->Error('Unable to create output file: '.$file);
    $this->_putheader();
}

public function Image($file, $x=null, $y=null, $w=0, $h=0, $type='', $link='')
{
    if(!isset($this->images[$file]))
    {
        // Retrieve only metadata
        $a = getimagesize($file);
        if($a===false)
            $this->Error('Missing or incorrect image file: '.$file);
        $this->images[$file] = array('w'=>$a[0], 'h'=>$a[1], 'type'=>$a[2], 'i'=>count($this->images)+1);
    }
    parent::Image($file,$x,$y,$w,$h,$type,$link);
}

public function Output($dest='', $name='', $isUTF8=false)
{
    if($this->state<3)
        $this->Close();
}

protected function _endpage()
{
    parent::_endpage();
    // Write page to file
    $this->_putstreamobject($this->pages[$this->page]);
    unset($this->pages[$this->page]);
}

protected function _getoffset()
{
    return ftell($this->f);
}

protected function _put($s)
{
    fwrite($this->f,$s."\n",strlen($s)+1);
}

protected function _putimages()
{
    foreach(array_keys($this->images) as $file)
    {
        $type = $this->images[$file]['type'];
        if($type==1)
            $info=$this->_parsegif($file);
        elseif($type==2)
            $info=$this->_parsejpg($file);
        elseif($type==3)
            $info=$this->_parsepng($file);
        else
            $this->Error('Unsupported image type: '.$file);
        $this->_putimage($info);
        $this->images[$file]['n'] = $info['n'];
        unset($info);
    }
}

protected function _putpage($n)
{
    $this->_newobj();
    $this->_put('<</Type /Page');
    $this->_put('/Parent 1 0 R');
    if(isset($this->PageInfo[$n]['size']))
        $this->_put(sprintf('/MediaBox [0 0 %.2F %.2F]',$this->PageInfo[$n]['size'][0],$this->PageInfo[$n]['size'][1]));
    if(isset($this->PageInfo[$n]['rotation']))
        $this->_put('/Rotate '.$this->PageInfo[$n]['rotation']);
    $this->_put('/Resources 2 0 R');
    if(!empty($this->PageLinks[$n]))
    {
        $s = '/Annots [';
        foreach($this->PageLinks[$n] as $pl)
            $s .= $pl[5].' 0 R ';
        $s .= ']';
        $this->_put($s);
    }
    if($this->WithAlpha)
        $this->_put('/Group <</Type /Group /S /Transparency /CS /DeviceRGB>>');
    $this->_put('/Contents '.(2+$n).' 0 R>>');
    $this->_put('endobj');
    // Link annotations
    $this->_putlinks($n);
}

protected function _putpages()
{
    $nb = $this->page;
    $n = $this->n;
    for($i=1;$i<=$nb;$i++)
    {
        $this->PageInfo[$i]['n'] = ++$n;
        foreach($this->PageLinks[$i] as &$pl)
            $pl[5] = ++$n;
        unset($pl);
    }
    for($i=1;$i<=$nb;$i++)
        $this->_putpage($i);
    // Pages root
    $this->_newobj(1);
    $this->_put('<</Type /Pages');
    $kids = '/Kids [';
    for($i=1;$i<=$nb;$i++)
        $kids .= $this->PageInfo[$i]['n'].' 0 R ';
    $kids .= ']';
    $this->_put($kids);
    $this->_put('/Count '.$nb);
    if($this->DefOrientation=='P')
    {
        $w = $this->DefPageSize[0];
        $h = $this->DefPageSize[1];
    }
    else
    {
        $w = $this->DefPageSize[1];
        $h = $this->DefPageSize[0];
    }
    $this->_put(sprintf('/MediaBox [0 0 %.2F %.2F]',$w*$this->k,$h*$this->k));
    $this->_put('>>');
    $this->_put('endobj');
}

protected function _putheader()
{
    if($this->_getoffset()==0)
        parent::_putheader();
}

protected function _enddoc()
{
    parent::_enddoc();
    fclose($this->f);
}
}
?>