在PHP中，如何在评估字符串变量之前将其替换？

Question

Is it possible to expand/substitute a variable before PHP tries to evaluate its truth value?

I'm trying to code a single Wordpress template that will execute different queries depending on which page we're on. If we're on the homepage, the query should look like this:

while ( $postlist->have_posts() ) : $postlist->the_post();
    // code...

If we're not on the homepage, the query should look like this:

while ( have_posts() ): the_post();
    // code...

So I thought I would try this:

$query_prefix = ( is_front_page() ) ? '$postlist->' : '';

$query_condition = $query_prefix.'have_posts()';
$query_do        = $query_prefix.'the_post()';

while ( $query_condition ): $query_do;
    // code...

The problem is, this is creating an infinite loop, because $query_condition is a string and evaluates to TRUE. It seems like PHP never 'reads' the content of the variable. I need my variable to expand itself literally, and only then offer itself for evaluation. Can anyone please tell me how how to do this?

Answer 1

Any of these answers work, but to provide another alternative:

if(is_front_page()) {
    $callable_condition = array($postlist,'have_posts');
    $callable_do = array($postlist,'the_post');
} else {
    $callable_condition = 'have_posts';
    $callable_do = 'the_post';
}

while(call_user_func($callable_condition)) : call_user_func($callable_do);

Also, if you are inside an object, you can use array($this,'method') to call a method of your object.

Answer 2

One way to handle this would to use a logical or statement in your while condition to loop based on different objects depending on the result of is_front_page() , and then an if statement to control the call to the_post() as well.

// loop while the front page and $postlist OR not the front page and not $postlist
while ( (is_front_page() && $postlist->have_posts() ) || ( !is_front_page() && have_posts() ) ): 
    // use $postlist if on the front page
    if ( is_front_page() && !empty($postlist) ){
        $postlist->the_post(); 
    } else { 
        the_post();
    }
    // the rest of your code
endwhile;

Answer 3

May be such example could possible help you. This is about using variables of variables :

class A {
    public function foo(){
        echo "foo" ;
    }
}

$a = new A() ;

$obj = 'a' ;
$method = "foo" ;


${$obj}->$method() ; //Will echo "foo"

Answer 4

I've always used the_title for determining the page.

$isHomePage = false;
if(the_title( '', '', FALSE ) == "Home")
{
    $isHomePage = true;
}

Then i use $isHomePage as a flag for anything else I need later in the page. This can be changed to look for any page you want to single out. It can get hairy if you have long page names though, so there is that.