PHP 输出session 验证码与图片不同步，图片总是快一步，求解！解决方案

<?
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;
?>

<img src="captcha.php" width="60" height="22" border="1" onclick= "this.src='captcha.php?act=captcha&'+Math.random()" style="cursor: pointer; vertical-align:middle" title="看不清？点击更换！" />

<?php
/**
 * 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) {
            $this->font_color = imagecolorallocate($this->img, rand(0,156), rand(0,156), rand(0,156));
        } else {
            $this->font_color = imagecolorallocate($this->img, hexdec(substr($this->font_color, 1,2)), hexdec(substr($this->font_color, 3,2)), hexdec(substr($this->font_color, 5,2)));
        }
        //设置背景色
        $background = imagecolorallocate($this->img,hexdec(substr($this->background, 1,2)),hexdec(substr($this->background, 3,2)),hexdec(substr($this->background, 5,2)));
        //画一个柜形，设置背景颜色。
        imagefilledrectangle($this->img,0, $this->height, $this->width, 0, $background);
        $this->creat_font();
        $this->creat_line();
        $this->output();
    }
    
    /**
     * 生成文字
     */
    private function creat_font() {
        $x = $this->width/$this->code_len;
        for ($i=0; $i<$this->code_len; $i++) {
            imagettftext($this->img, $this->font_size, rand(-30,30), $x*$i+rand(0,5), $this->height/1.4, $this->font_color, $this->font, $this->code[$i]);
            if($i==0)$this->x_start=$x*$i+5;
        }
    }
    
    /**
     * 画线
     */
    private function creat_line() {
        imagesetthickness($this->img, 3);
        $xpos   = ($this->font_size * 2) + rand(-5, 5);
        $width  = $this->width / 2.66 + rand(3, 10);
        $height = $this->font_size * 2.14;
    
        if ( rand(0,100) % 2 == 0 ) {
          $start = rand(0,66);
          $ypos  = $this->height / 2 - rand(10, 30);
          $xpos += rand(5, 15);
        } else {
          $start = rand(180, 246);
          $ypos  = $this->height / 2 + rand(10, 30);
        }
    
        $end = $start + rand(75, 110);
    
        imagearc($this->img, $xpos, $ypos, $width, $height, $start, $end, $this->font_color);
        
        if ( rand(1,75) % 2 == 0 ) {
          $start = rand(45, 111);
          $ypos  = $this->height / 2 - rand(10, 30);
          $xpos += rand(5, 15);
        } else {
          $start = rand(200, 250);
          $ypos  = $this->height / 2 + rand(10, 30);
        }
    
        $end = $start + rand(75, 100);
    
        imagearc($this->img, $this->width * .75, $ypos, $width, $height, $start, $end, $this->font_color);
    }
    
    /**
     * 输出图片
     */
    private function output() {
        header("content-type:image/png\r\n");
        imagepng($this->img);
        imagedestroy($this->img);
    }
}