yii verification code cannot be displayed

No display: (Recommended learning: yii tutorial )

## First guarantee your controler The captcha method inside is accessible and assigned permissions, which are set in the rule.

Second, ensure that your PHP GD plug-in has been enabled

Third, if it still does not display, then try this method , someone else told me.

ob_clean();//加上这行

header('Pragma: public'); 
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0'); 
header('Content-Transfer-Encoding: binary'); 
header("Content-type: image/png"); 
imagepng($image);
imagedestroy($image);

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