未定义索引 HTTP_HOST 即使已检查

Question

Here's the code:

if (isset($_SERVER['HTTP_HOST']) === TRUE) {
  $host = $_SERVER['HTTP_HOST'];
}

How is it possible to get an "Undefined index HTTP_HOST" on the line inside the if statement? I mean, the index setting is checked before it is used.

And why could the HTTP_HOST sometimes be not set?

Answer 1

Are you using PHP-CLI?

HTTP_HOST works only on the browser.

Answer 2

An bad implemented browser can omit send que host header information, try this example:

telnet myphpserver.com 80
> GET / <enter><enter>

in this case $_SERVER['HTTP_HOST'] not have assigned value, in this case u can use $_SERVER['SERVER_NAME'] but only if $_SERVER['HTTP_HOST'] is empty, because no is the same.

Answer 3

The HTTP_HOST must always be set if you are running on a browser... then there is no need to check... simply,

$host = $_SERVER['HTTP_HOST'];

is enough

Answer 4

I would normally omit the === TRUE , as it's not needed here because isset() returns a boolean, but that shouldn't stop your code from working.

I would also set $host to a sensible default (depends on your application) before the if statement. I have a general rule to not introduce a new variable inside a conditional if it's going to be referred to later.

$host = FALSE;    // or $host = ''; etc. depending on how you'll use it later.
if (isset($_SERVER['HTTP_HOST'])) {
  $host = $_SERVER['HTTP_HOST'];
}

Answer 5

When a request is done with an empty host:

GET / HTTP/1.1
Host:

Then isset($_SERVER['HTTP_HOST']) is true!

It is better to use empty like:

$host = '';
if (!empty($_SERVER['HTTP_HOST'])) {
  $host = $_SERVER['HTTP_HOST'];
}

For detailed info take a look here https://shiflett.org/blog/2006/server-name-versus-http-host