如何使这个PHP if语句更简单，更短？

I have feeling that the following code can be shortened but have no idea how to rewrite it :)

$post_type = isset( $_GET['post_type'] ) ? $_GET['post_type'] : '';

if( $post_type == 'faq' && !isset( $_REQUEST['mode'] ) )
  $_REQUEST['mode'] = 'excerpt';

// if I just go with else... it doesn't work
elseif( $post_type !== 'faq' && !isset( $_REQUEST['mode'] ) )
  $_REQUEST['mode'] = 'list';

Update

As you can see, !isset( $_REQUEST['mode'] ) is used twice. $post_type == 'faq' is also used twice, but in the elseif I'm checking negative.

Seriously there is no way to rewrite it to one if without else...?

How about using a ternary operator for testing 'faq' ?

$post_type = isset( $_GET['post_type'] ) ? $_GET['post_type'] : '';
if (!isset( $_REQUEST['mode'] )) {
    $_REQUEST['mode'] = ($post_type == 'faq') ? 'excerpt' : 'list';
}

maybe use more intermediate variables, to improve readability of the if statements.

$post_type = isset( $_GET['post_type'] ) ? $_GET['post_type'] : '';
$notRequestMode=!isset( $_REQUEST['mode'] );

if( $post_type == 'faq' &&  $notRequestMode)
  $_REQUEST['mode'] = 'excerpt';

// if I just go with else... it doesn't work
elseif( $post_type !== 'faq' && $notRequestMode)
  $_REQUEST['mode'] = 'list';

There is a good chance you have a lot of $post_type, so I would first set that within a switch:

switch($post_type) {
case 'faq':
  // code here
  break;

case 'somethingElse':
  // code here
  break;

// etc.

From there, you could even nest another switch in for $_REQUEST['mode'] if appropriate, or just include your if statements.

As a note on formatting, if I have a lot of conditions, I will put those conditions on multiple lines:

if (
  $post_type == 'faq' && 
  !isset( $_REQUEST['mode'] ) && 
  // more conditions here
) {

Since you want short, here's short based on your example code.

if(!isset( $_REQUEST['mode'] )){
$_REQUEST['mode'] = 'list';
if($_GET['post_type']=="faq")
$_REQUEST['mode'] = 'excerpt';
} 

Your actual code can be resolved to:

if (!isset($_REQUEST['mode'])) {
  if (isset($_GET['post_type'])) {
    $_REQUEST['mode'] = ($_GET['post_type'] == 'faq') ? 'excerpt' : 'list';
  }
}

But is it really better? I mean, you're surely going back to your code some time in the future...

Try this

$post_type = isset( $_GET['post_type'] ) ? $_GET['post_type'] : '';
$_REQUEST['mode'] = !isset($_REQUEST['mode'])?(($post_type==='faq')?'excerpt':'list'):'';

In PHP 5.3+ you can use the shortened ternary:

$post_type = $_GET['post_type'] ?: '';
if (!isset( $_REQUEST['mode'] )) {
  $_REQUEST['mode'] = $post_type == 'faq' ? 'excerpt' : 'list';
}

But the shortest is probably:

if (!isset( $_REQUEST['mode'] )) {
  $_REQUEST['mode'] = isset($_GET['post_type']) && $_GET['post_type'] == 'faq' ? 'excerpt' : 'list';
}

Or on a single line with the shortened ternary:

$_REQUEST['mode'] = $_REQUEST['mode'] ?: (isset($_GET['post_type']) && $_GET['post_type'] == 'faq' ? 'excerpt' : 'list');