<?php

$test = array(
	(object) [
		'pais' => 'Brasil',
		'principal' => 1,
	],
	(object) [
		'pais' => 'Brasil',
		'principal' => 0,
	],
	(object) [
		'pais' => 'Brasil',
		'principal' => 1,
	],
	(object) [
		'pais' => 'Brasil',
		'principal' => 0,
	]
);
$keys = [];
foreach($test as $k => $v) {
	if($v->principal === 1)
		$keys[] = $k;
}
print_r($keys);