The application of PHP class solution

Application of PHP class
I am a novice just contact with the concept of class, now I found on the Internet a code to generate a class, how to output a verification code where the output of the code generated in the class? The code in the class is like this:
Class Authcode
{
var $image;
var $sBgcolor;
var $nWidth;
var $nHeight;
var $nLen;
var $bNoise;
var $nNoise;
var $bBorder;
var $aFontlist;
function Authcode ()
{
$this->sbgcolor = "#FFFFFF";
$this->nwidth = 70;
$this->nheight = 25;
$this->nleftmargin = 5;
$this->nrightmargin = 5;
$this->ntopmargin = 3;
$this->nbottommargin = 2;
$this->nlen = 4;
$this->bnoise = true;
$this->nnoisepoint = 50;
$this->nnoiseline = 5;
$this->bborder = true;

$this->afontlist = "Arial.ttf";
}

function outputimg ()
{
$this->image = "";
$this->image = imagecreate ($this->nwidth, $this->nheight);
$back = $this->getcolor ($this->sbgcolor);
Imagefilledrectangle ($this->image, 0, 0, $this->nwidth, $this->nheight, $back);
$size = ($this->nwidth-$this->nleftmargin-$this->nrightmargin)/$this->nlen;
if ($size > ($this->nheight-$this->ntopmargin-$this->nbottommargin))
$size = $this->nheight-$this->ntopmargin-$this->nbottommargin;

$left = ($this->nwidth-$this->nlen* ($size + $size/10)/2 + $this->nleftmargin;
$code = "";
for ($i =0; $i < $this->nlen; $i + +)
{
$randtext = rand (0, 9);
$code. = $randtext;
$textColor = Imagecolorallocate ($this->image, rand (0, +), rand (0, +), rand (0, 100));
$font = $this->afontlist;
$randsize = rand ($size-$size/10, $size + $size/10);
$location = $left + ($i * $size + $size/10);
Imagettftext ($this->image, $randsize, Rand ( -18,18), $location, Rand ($size, $size + $size/5) + $this->ntopmargin, $ TextColor, $font, $randtext);
}
if ($this->bnoise = = True) $this->setnoise ();
$_session[' yzm '] = MD5 ($code);
$bordercolor = $this->getcolor ("#FFFFFF");
if ($this->bborder==true) imagerectangle ($this->image, 0, 0, $this->nwidth-1, $this->nheight-1, $ BorderColor);
Header ("Content-type:image/png");
Imagepng ($this->image);
Imagedestroy ($this->image);
}
function setnoise ()//Set the noise point
{
for ($i =0; $i < $this->nnoiseline; $i +) {
$randColor = Imagecolorallocate ($this->image, rand (0, 255), rand (0, 255), rand (0, 255));
Imageline ($this->image, rand (0, $this->nwidth), rand (0, $this->nheight), rand (0, $this->nwidth), rand (0, $ this->nheight), $randColor);
}

for ($i =0; $i < $this->nnoisepoint; $i +) {
$randColor = Imagecolorallocate ($this->image, rand (0, 255), rand (0, 255), rand (0, 255));
Imagesetpixel ($this->image, rand (0, $this->nwidth), rand (0, $this->nheight), $randColor);

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