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);    }}