是否可以通过$ _GET获取空数组值？

Is it possible to get empty array(array with 0 items) value via $_GET?

Is it possible with direct value setting?

count($_GET['param'])==0

If array is empty or containing values that doesn't matters. Just declare a variable and pass the $_GET[] to the variable.

for example,

$para=$_GET['param'];

and now

if(is_array($para))
{
//  

} 
else{
$para=new array();
}

You just need an empty value url = myform.php?param=&param2=

In form just let the value blank:

<input type='text' name='param' value ='' />

For an empty array:

url: myform.php?param[]=&param2[some_key]=

in form: <input type='text' name='param[]' value ='' />

From Ajax: (I remember this was so anti-intuitive and hard to search for):

ajax{
    ...
    data: {'params[]':'','params2[some_key]':''}
}

Workaround: Just edit the back-end and if there is no data for params or it is not an array (null, empty whatever ..) just assign an empty string to it:

$param = (isset($_GET['param']) && is_array($_GET['param']))? $_GET['param'] : array();

Update:
I did few tests and it seems there is no way to put "nothing" in the request ussing a form or ajax.
0, '', Null are valid values for the $_GET but empty array is not even created. So to answer your question, it is NOT possible to get empty array value from the front-end.

There are few options to edit $_GET manually in the back-end:

<?php
    if(!isset($_GET['param']) || !$_GET['param']){ //not set or (null,0,"")
        $_GET['param'] = array();
    }
    if(count($_GET['param'])==0){...}; // 0 if no 'param' was provided.

passing empty array via GET is not possible under normal situation. That said, I can think of a really rare way that the checking you used will return true.

http://domain.com/receiver?param=a%3A0%3A%7B%7D

The above is basically a serialized and urlencoded empty array with the key 'param'

if the target server have some sort of filter that auto unserialize all incoming data, then it might happen. (or something like the below)

foreach($_GET as $key => $value){
$_GET[$key] = unserialize($value);
}

count($_GET['param'])==0

I know this is a far fetch but it is possible, maybe some private test server that only handles serialized data but accidentally open to public e.t.c. That said, it is still only passing a serialized empty array instead of a empty array itself. But let's be honest, this answer is more like a joke/fun answer that tries to point out under some very rare case

count($_GET['param'])==0

will return true (Without actively assigning values @ server side)