What are the precise rules/PHP function for encoding strings into POST arrays?

Question

Greetings,

Just getting into PHP web development. I've got an HTML form where a user checks some series of dynamically-generated checkboxes, and submits via POST. On the PHP side, I want to check which of the check-boxes were clicked.

I have an array $full_list, and am doing something like

$selected_checkboxes = array_filter($full_list, function($item) {
    array_key_exists($item, $_POST);
  }

I run into problems when a list item is named, for example "Peanut Butter", since in the POST array it is named "Peanut_Butter".

I could certainly just str_replace " " with "_" before checking array_key_exists, but I imagine that there is a more fundamental encoding problem here; specifically, I'm not sure of exactly what layer transforms normal strings in HTML Forms (value="Peanut Butter") into "Peanut_Butter".

So:

• what layer is responsible for this conversion? Is it the browser?
• what are the exact conversion rules, and is there a PHP function out there that will replicate that exact conversion?

Thanks!