Passing $_POST/$_GET as a parameter to function

I came across an interesting piece of PHP code which has me a bit stumped as to why the author has chosen to do this.

function do_something($db, $post_vars){
    foreach($post_vars as $key => $value{
        $vars[$key] = mysqli_real_escape_string($db, $value);
    }
    return $vars;
}

$db = mysqli_connect("myhost","myuser","mypassw","mybd") or die("Error " . mysqli_error($link)); 
do_something($db, $_POST);

It got me thinking about why someone would want to pass $_POST as a variable and just not access it directly inside the function? The only benefit I could think of (and this was a bit of a long shot) was if we were to append other information to $_POST before calling the function (such as):

function do_something($db, $post_vars){
    foreach($post_vars as $key => $value{
        $vars[$key] = mysqli_real_escape_string($db, $value);
    }
    return $vars;
}

$db = mysqli_connect("myhost","myuser","mypassw","mybd") or die("Error " . mysqli_error($link)); 

foreach($_POST as $post_key => $post_value){
    $post[$post_key] = $post_value;
}

$post['my_custom_var'] = "a";

do_something($db, $post);

There is, however, no evidence of this practise anywhere in the code. Just calls to do_something() with $_POST being passed as an arugment.

My question then is, is there any benefit in doing it like this that I’ve missed or did the author simply not understand that $_POST is a global variable?

A complete long shot: Is there perhaps even any well intended “later additions” they could make to this (such as my example) that would almost justify this practise or is this just a case of misunderstanding. Or perhaps is there a security implication that could justify the practise?

Answer

IMHO it’s a practice of abstraction, and there are benefits:

1. Generality: by receiving $_POST as a parameter, the function becomes less tightly coupled to $_POST. The function may serve more scenarios & possibly be more reusable.

2. Inversion of control: because the function’s dependency($_POST) is injected from outside, you have more control over the function. It is a bit unlikely but let’s suppose your form has been updated and now you need to submit via GET method. Without modifying the function’s body, passing in $_GET on the caller’s side, is enough to reflect the change.

3. Test fixture isolation: To mock FORM inputs to test a certain code path in the function, it is better access global states (such as $_POST) in an abstracted way, so that the test itself does not bring side effects to other part of the system.