php验证码图片不显示图片怎么办

<form action="signin.php" method="post" class="col-xs-12 col-sm-4" id="post-form">
    <h2>登录</h2>
    <div class="form-group">
         <label for="username">用户名</label>
         <input type="text" class="form-control" id="username" name="username">
    </div>
    <div class="form-group">
         <label for="password">密码</label>
         <input type="password" class="form-control" id="password" name="password">
    </div>
    <div class="form-group row">
         <div class="col-xs-5">
              <label class="sr-only">验证码：</label>
              <input type="text" class="form-control" id="authcode" name="authcode" placeholder="验证码">
         </div>
         <div class="col-xs-7">
              <img class="authcod_img" src="authcode.php"onclick="this.src='authcode.php?t='+Math.random()" name="authcode">
         </div>
     </div>
     <button type="button" class="btn btn-default">提交</button>
</form>
<?php
session_start();
 
header('Content-Type: image/png');
 
$img = imagecreatetruecolor(150, 40);
$bgcolor = imagecolorallocate($img, rand(0,100),rand(0,100),rand(0,100));
imagefill($img, 0,0,$bgcolor);

ob_clean();
 
header('Content-Type: image/png');