Limit WP code scope into plugin

In PHP, constants are global so they have no scope. Per the PHP Manual:

Like superglobals, the scope of a constant is global. You can access
constants anywhere in your script without regard to scope.

There is an exception to this if you are using a PHP Class. You can then tie the constant to that class.

class My_Class
{
    const BLAH = 'This is a Constant';
{
print My_Class::BLAH

As for functions they, like constants, are global as well. Per the PHP Manual:

All functions and classes in PHP have the global scope – they can be
called outside a function even if they were defined inside and vice
versa.

For example, if I have:

function my_func() {
    function my_other_func() {
        // Code
    }

    // More Code
}

I can call my_other_func() on its own even though it is being defined within my_func().

You can again skirt this using a PHP Class:

class My_Class
{
   public function echo_hello_world() {
       $this->echo_hello();
       $this->echo_world();
   }

   private function echo_hello() {
       echo 'Hello ';
   }

   private function echo_world() {
       echo 'World!';
   }
}

$testObj = new My_Class();

$testObj->echo_hello_world();

However, if classes are overkill, your best bet is to simply namespace everything by appending something unique in front of the names you are currently using. Usually I will append the textdomain I defined for the plugin/theme so my_function() becomes textdomain_my_function().

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