About PHP using System () to open Firefox under Linux, error: Error:no display specified, how to handle

About PHP using System () to open Firefox under Linux, error: Error:no display specified
Will you please help the heroes?

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Do you have permission to operate?

Where's the code?
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Browser access and access under the terminal are two permissions. The browser is the normal user rights, you install sudo on the machine
#查询sudo版本
Apt-cache Search sudo
#安装sudo
Apt-get Install sudo
Then: System ("sudo firefox http://www.baidu.com"); Switch to root permissions

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Explore

The whole code is this:
$cmd = "sudo firefox--saveimage http://baidu.com";
System ($cmd, $STR);
Echo $str;
What I want to achieve is to open Firefox to save the entire page as a picture

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