PHP输出session验证码与图片不同步，图片老是快一步，求解

session_start();

function random($len)

{

$srcstr="ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

mt_srand();

$strs="";

for($i=0;$i<$len;$i++){

$strs.=$srcstr[mt_rand(0,35)];

}

return strtoupper($strs);

}

$str=random(4); 

$width = 50; 

$height = 25; 

@header("Content-Type:image/png");

$_SESSION["captcha"] = $str;

//echo $str;

$im=imagecreate($width,$height);

$back=imagecolorallocate($im,0xFF,0xFF,0xFF);

$pix=imagecolorallocate($im,187,230,247);

$font=imagecolorallocate($im,41,163,238);

mt_srand();

for($i=0;$i<1000;$i++)

{

imagesetpixel($im,mt_rand(0,$width),mt_rand(0,$height),$pix);

}

imagestring($im, 5, 7, 5,$str, $font);

imagerectangle($im,0,0,$width-1,$height-1,$font);

imagepng($im);

imagedestroy($im);

$_SESSION["captcha"] = $str;

?>

/**

 * 生成验证码

 * 类用法

 * $checkcode = new checkcode();

 * $checkcode->doimage();

 * //取得验证

 * $_SESSION['code']=$checkcode->get_code();

	session_start();

	include './checkcode.class.php';

	$checkcode = new checkcode('C:\WINDOWS\Fonts\ARIAL.TTF');

	$checkcode->doimage();

	$_SESSION['code']=$checkcode->get_code();

 */

class checkcode {

	//验证码的宽度

	public $width=130;

	

	//验证码的高

	public $height=50;

	

	//设置字体的地址

	private $font;

	

	//设置字体色

	public $font_color;

	

	//设置随机生成因子

	public $charset = 'abcdefghkmnprstuvwyzABCDEFGHKLMNPRSTUVWYZ23456789';

	

	//设置背景色

	public $background = '#EDF7FF';

	

	//生成验证码字符数

	public $code_len = 4;

	

	//字体大小

	public $font_size = 20;

	

	//验证码

	private $code;

	

	//图片内存

	private $img;

	

	//文字X轴开始的地方

	private $x_start;

		

	function __construct($fontpath) {

		$this->font =$fontpath;

	}

	/**

	 * 生成随机验证码。

	 */

	protected function creat_code() {

		$code = '';

		$charset_len = strlen($this->charset)-1;

		for ($i=0; $i<$this->code_len; $i++) {

			$code .= $this->charset[rand(1, $charset_len)];

		}

		$this->code = $code;

	}

	

	/**

	 * 获取验证码

	 */

	public function get_code() {

		return strtolower($this->code);

	}

	

	/**

	 * 生成图片

	 */

	public function doimage() {

		$code = $this->creat_code();

		$this->img = imagecreatetruecolor($this->width, $this->height);

		if (!$this->font_color) {