Method not working outside of the class it is in - php

Question

Functions are not working due to undefined variables and I have no clue why that is. I think I just don't get the scope at which objects/classes work. Do I need to make a reference like $obj from which to call the functions? If so why is it not working in this example?
<?php
class employee {
// Properties
public $name;
public $salary = 0;
// Methods
function set_name($name) {
$this->name = $name;
}
function get_name() {
return $this->name;
}
function set_salary($salary) {
$this->salary = $salary;
}
function get_salary() {
return $this->salary;
}
function populate()
{
$worker = array();
$worker[0] = new employee();
$worker[0]->set_name('Linda');
$worker[0]->set_salary(5800);
$worker[1] = new employee();
$worker[1]->set_name('Manuel');
$worker[1]->set_salary(2750);
$worker[2] = new employee();
$worker[2]->set_name('Luis');
$worker[2]->set_salary(3200);
$worker[3] = new employee();
$worker[3]->set_name('Carly');
$worker[3]->set_salary(2999);
$i = 0;
}
function checkTaxes()
{
while ($i < 4)
{
echo "Name: " . $worker[$i]->get_name();
echo " - Salary: " . $worker[$i]->get_salary();
if ($worker[$i]->get_salary() >= 3000)
{
echo " - You have to pay taxes!<br><br>";
}
else
{
echo " - You don't have to pay taxes!<br><br>";
}
$i++;
}
}
}
$obj = new employee;
$obj->populate();
$obj->checkTaxes();
?>

Answer 1

$worker is undefined in checkTaxes()
If you want to use $worker in checkTaxes() or other functions, you should define $worker in class employee with private $worker = [];, just like $name and $salary.And after that, you can update it in populate() with $this->worker[0] = xxx;, and read it in checkTaxes() with $this->worker[$i]->getxxx.