Online PHP editor | output for Sn1M6

<?php class SomeHelperClass { public $_arr = []; public function addValue($value, $key = null): void { $this->_arr[$key ?? ((array_key_last($this->_arr) ?? -1) + 1)] = $value; } } $test = new SomeHelperClass; var_export($test->_arr); echo "\n"; $test->addValue(1); var_export($test->_arr); echo "\n"; $test->addValue(2); var_export($test->_arr); echo "\n"; $test->addValue(3, 4); var_export($test->_arr); echo "\n"; $test->addValue(4, 3); var_export($test->_arr); echo "\n"; $test->addValue(5); var_export($test->_arr);