为什么这会返回0？

I'm trying to make a custom captcha generator. In short, the server returns a URL of an image through AJAX and keeps in session storage an association with the image.

PHP:

add_action( 'wp_ajax_set_animal_captcha', 'set_animal_captcha' );
$capdir = get_template_directory_uri() . '/assets/captcha/';
$capmap = array ( 'cat'  => $capdir . 'Captcha_Cat.png',
                  'dog'  => $capdir . 'Captcha_Dog.png',
                  'fish' => $capdir . 'Captcha_Fish.png' );
function set_animal_captcha ( )
{
    // returns image url of random animal and stores in session storage
    // a reference to that animal
    $randAnimal = array_rand($capmap,1);
    $_SESSION['randAnimal'] = $randAnimal;
    die(json_encode($capmap[$randAnimal]));
}

JS:

function capHandler ( imgid )
{
    // imgid: id of the image 

    this.imgid = imgid;
    this.formData = new FormData();
    this.formData.append('action', 'set_animal_captcha');
    this.set = function ( )
    {
        $.ajax({
            url: ajaxurl,
            type: 'POST',
            async: false,
            success: function ( animalUrl ) { alert(animalUrl); },
            error: function ( ) { alert("Error in getting captcha image") },
            cache: false,
            contentType: false,
            processData: false
        });
    }
}

My problem is that it's alerting

0

and I can't figure out why. Because I know the success function is being called, there must be something wrong with the PHP.

Why are you setting $capmap outside of set_animal_captcha()? If you really must keep it outside set_animal_captcha, then set $capmap as a global inside your function. Like this:

function set_animal_captcha ( )
{
    global $capmap;
    // returns image url of random animal and stores in session storage
    // a reference to that animal
    $randAnimal = array_rand($capmap,1);
    $_SESSION['randAnimal'] = $randAnimal;
    die(json_encode($capmap[$randAnimal]));
}

Please die the statement from where you are returning some value by putting this:

 die();