Why, if a function accepts arguments, it fails on ajax calls?

Question

I am trying to write a WP function to work with both ajax and direct calls, something like this:

PHP

function some_function($var){
    $var = !empty($var) ? $var : $_POST['var'];
    echo $var;
    
    //or even

    $var = null;
    echo 'something';

    if(!empty($_POST['action'])) wp_die();
}

AJAX CALL

let ajaxurl = '███';
let data = {'action': 'somefunction','var':'foo')};
$.post(ajaxurl, data, function(response) {console.log(response);});

WP use

add_action( 'wp_ajax_somefunction', 'some_function',10,1);
add_action( 'wp_ajax_nopriv_somefunction', 'some_function',10,1);

Another WP use

some_function('bar');

However, any time I place $var as an accepted function argument, some_function($var), my ajax calls start returning a 500 error. So, something like this

function some_function(){
    $var = !empty($var) ? $var : $_POST['var'];
    echo $var;
}

works for ajax.

I tried looking up wp ajax & arguments, but the search results are always about the variables we pass through ajax, not the callback function arguments. The only thing I learned is that we have to add a number of accepted arguments into add_action()

What am I doing wrong?

Thank you.

...P.S. I found a funny workaround:

function some_function_ajax(){
    $var = $_POST['var'];
    some_function($var);
}
function some_function($var){
    echo $var;
} // =)

but still, what is the right way?

---

Jackob Peattie, thanks for the hint! So, the working code looks like this, then:

//JS (jQuery):
let data = {
   'action' : 'func',
   'var' : 'something',
   'nonce' : nonce
};
$.post(ajaxurl, data);

//add_action:
add_action( 'wp_ajax_func', 'func_ajax');
add_action( 'wp_ajax_nopriv_func', 'func_ajax');

//PHP:
function func_ajax(){
   //verify nonce
   if(empty($_POST['nonce']) || !wp_verify_nonce( $_POST['nonce'], 'myajax-nonce' ) ) wp_die();

    $var = $_POST['var'];
    func($var);

   //end ajax call
   wp_die();
}

function func($var){
    echo $var;
}

Answer 1

No arguments are passed to the AJAX callback function. Variables passed with the request are available in $_GET or $_POST. So if your function expects an argument, it can’t be used as the hooked callback directly.

Your workaround is the correct way to use a function that accepts arguments in an AJAX request.