If all your methods are in a class you can use the below approach whereby the variable is a class reference set in the class and so automatically available to all class methods.

Variables in the class are called properties.

class Submit extends \RightNow\Models\Base
{
    private $i = 0; // set class based variable;

    function __construct()
    {
        parent::__construct();
    }
    function inner () {
        return $this->i++;
    }
    function outer(){
       // $i = 0; You probably don't want to set this each call? 
        $this->inner();
        echo $this->i; 
    }
}

$submit = new Submit();
$submit->outer();

See above everything references the class property $i which is set in the class.

Alternatively if you're working with the same process over multiple varaibles from you can group them up into an array and simply pass the array as the single argument.

Your code will look slightly different depending on if the values are coming from inside or outside the class.

References :

PHP - passing variables to classes

https://www.php.net/manual/en/language.oop5.basic.php

https://www.php.net/manual/en/language.oop5.properties.php

You can pass a variable by reference. E.g.:

function inner (&$i) {
    return $i++;
}

function outer(){
    $i = 0;
    inner($i);
    echo $i; // 1
}

UPD

If you want to pass multiple variables:

1. you can pass an array by reference:

function inner (&$arr) {
   $arr['i']++;
   $arr['j']--;
}

function outer() {
   $a = array('i' => 5, 'j' => 10);
   inner($a);
   echo $a['i'] . "\n" . $a['j']; // 6 9
}

2. You can pass an object. In this case it always passed by reference implicitly. E.g.

function inner($obj) { // no & needed, because it is an object reference itself
    $obj->a++;
    $obj->b--;
}


function outer() {
    $obj = new \stdClass();
    $obj->a = 5;
    $obj->b = 10;
    inner($obj);
    echo $obj->a . "\n" . $obj->b;  // 6 9
}

3. inside the same class you can use private members, but make sure the functions are not recursive (i.e. the same field is not reused simultaneously in different places):

class Submit extends \RightNow\Models\Base
{
    private $_i;

    function __construct()
    {
        parent::__construct();
    }
    function inner () {
        $this->_i++;
    }
    function outer(){
        $this->_i = 0;
        $this->inner();
        echo $this->_i;
    }
}