How to skip internal calling function in PHPUnit test?

Question

I have a class Receipt.php that I need to PHPUnit test.

<?php
namespace TDD;
class Receipt {

    public $user_id = 1;
    private $pending_amount = 45;   

    
    public function total(array $items = []){
        $this->pending();   
        $items[] = $this->pending_amount;
        return array_sum($items);
    }

    public function tax($amount,$tax){
        return $amount * $tax;
    }
    
    private function pending()
    {
        
        $sql = 'select pending_amount from Pending_transtions where user_id =' . $this->user_id . ' limit 1;';
        //$pending_amt = $this->mainDb->get_sql_row($sql);
        //$this->pending = $pending_amt['pending_amount'];  
        $this->pending_amount = 55;
    }
    
    public function addTaxPending($tax){
        return $this->pending_amount * $tax;
    }
}
?>

In PHPUnit test file ReceiptTest.php

<?php
namespace TDD\Test;

require(dirname(dirname(__FILE__))).DIRECTORY_SEPARATOR.'vendor'.DIRECTORY_SEPARATOR.'autoload.php'; 
use PHPUnit\Framework\TestCase; // its imoprt core class 
use TDD\Receipt; //its load our receipt class




class ReceiptTest extends TestCase{
    public function setUp(): void {
        $this->Receipt = new Receipt('s','p');
    }
    public function tearDown(): void{
        unset($this->Receipt);
    }
    public function testTotal(){
        $input = [0,2,5,8];
        $output = $this->Receipt->total($input);
        $this->assertEquals(60,$output,"this is not valid");        
    }   

    public function testTax(){
        $inputAmount = 10.00;
        $inputTax =  0.10;
        $output = $this->Receipt->tax($inputAmount,$inputTax);
        $this->assertEquals(1.0,$output,"this tax expecting 1.0");      
    }   
    
}
?>

I want to avoid $this->pending(); internal calling function at function total() in Receipt class. But I need to access of property pending_amount.

Here my challenges are pending_amount private property. How can I handle that?

How can skip pending function and how to access pending_amount property on PHPUnit file?

I also tried this way

public function test_total2(){

        
        $myStubObject = $this
        ->getMockBuilder(Receipt::class)
        ->setConstructorArgs(['s','p'])     
        ->setMethods(["pending"])
        ->getMock();
  
      $myStubObject
        ->method('pending')
        ->withAnyParameters()
        ->willReturn(45);

        $input = [0,2,5,8];
        $output = $myStubObject->total($input);
        $this->assertEquals(60,$output);
    }

Its throwing an error like

1. TDD\Test\ReceiptTest::test_total2 PHPUnit\Framework\MockObject\MethodCannotBeConfiguredException: Trying to configure method >"pending" which cannot be configured because it does not exist, has not been specified, is final, >or is static

But there is return in Pending function so I need to access pending_amount property. I hope that I clearly explain my problem, If you have any doubts please don't hesitate.

I'll recorrect it. Looking for your valuable inputs