Host header information (new employee)

Host header information (for advice from new users) in_array ($ _ SERVER & nbsp; ['http _ host'], array ('www .baidu.com ') I know HOST & nbsp; does the header contain Baidu's url? When will the content of the $ _ SERVER variable be included ?, What Host header information is available (for new users)


In_array ($ _ SERVER ['http _ host'], array ('www .baidu.com '))


I know whether the Host header information contains Baidu's URL. but what does it mean?
When will the content of the $ _ SERVER variable be included?
------ Solution --------------------
What's the point? look at your program.

$ _ SERVER is a super global variable. it exists when you access the script.