<?php
class example
{
    public $array;

    public function __construct()
    {
        $this->array = array();

    }

    public function do_work()
    {
        $this->array[] = 'test';
    }
}
$test = new example();
$test->do_work();
echo $test->array . "\n";
var_dump($test->array);

Array
array(1) {
  [0]=>
  string(4) "test"
}