PHP验证码图片不能输出,该如何解决

function _code(){
//运行验证码
    //随记码个数
    $_rnd_code=4;
    for($i=0;$i<$_rnd_code;$i++){
        $_nmsg.=dechex(mt_rand(0, 15));
    }
    //保存在session中
    $_SESSION['code']=$_nmsg;
    //长和高
    $_width=75;
    $_height=25;
    //创建一张图片
    $_img=imagecreatetruecolor($_width,$_height);
    //白色
    $_white=imagecolorallocate($_img, 255, 255, 255);
    //填充
    imagefill($_img, 0, 0, $_white);
    $_flag=false;
    if($_flag){
    //黑色,边框
        $_black=imagecolorallocate($_img, 0,0,0);
        imagerectangle($_img, 0,0, $_width-1,$_height-1,$_black);
    }
    //画出6个线条
    for($i=0;$i<6;$i++){
        $_rnd_color=imagecolorallocate($_img, mt_rand(0,255), mt_rand(0,255), mt_rand(0,255));
        imageline($_img,mt_rand(0,$_width),mt_rand(0,$_height),mt_rand(0,$_width),mt_rand(0,$_height), $_rnd_color);
    }
    //随即雪花
    for($i;$i<100;$i++){
        $_rnd_color=imagecolorallocate($_img,mt_rand(200,255), mt_rand(200,255), mt_rand(200,255));
        imagestring($_img,1, mt_rand(1, $_width),mt_rand(1,$_height), '*', $_rnd_color);    
    }
    //

_code();

    //随记码个数
    $_rnd_code=4;
    for($i=0;$i<$_rnd_code;$i++){
        $_nmsg.=dechex(mt_rand(0, 15));
    }
    //保存在session中
    $_SESSION['code']=$_nmsg;
    //长和高
    $_width=75;
    $_height=25;
    //创建一张图片
    $_img=imagecreatetruecolor($_width,$_height);
    //白色
    $_white=imagecolorallocate($_img, 255, 255, 255);
    //填充
    imagefill($_img, 0, 0, $_white);
    $_flag=false;
    if($_flag){
    //黑色,边框
        $_black=imagecolorallocate($_img, 0,0,0);
        imagerectangle($_img, 0,0, $_width-1,$_height-1,$_black);
    }
    //画出6个线条
    for($i=0;$i<6;$i++){
        $_rnd_color=imagecolorallocate($_img, mt_rand(0,255), mt_rand(0,255), mt_rand(0,255));
        imageline($_img,mt_rand(0,$_width),mt_rand(0,$_height),mt_rand(0,$_width),mt_rand(0,$_height), $_rnd_color);
    }
    //随即雪花
    for($i;$i<100;$i++){
        $_rnd_color=imagecolorallocate($_img,mt_rand(200,255), mt_rand(200,255), mt_rand(200,255));
        imagestring($_img,1, mt_rand(1, $_width),mt_rand(1,$_height), '*', $_rnd_color);    
    }
    //

------解决方案--------------------