This works for me (I edited my code a little to show):

// First log in
$url = 'http://[your-domain]/rest/V1/integration/admin/token';
$body = '{"username":"user", "password":"pass"}';
$header = 'Content-type: application/json';
$options = [    'method' => 'POST',
                'content' => $body,
                'header' => $header
           ];
$context = stream_context_create(['http' => $options]);
$token = json_decode(file_get_contents($url, false, $context));

// Some settings

$header = 'Authorization: Bearer ' . $token . "\r\n"
    . 'Content-type: application/json';
$options = [    'method' => 'GET',
                'content' => $body,
                'header' => $header
           ];
$context = stream_context_create(['http' => $options]);

$url = 'http://outpro.lt/index.php/rest/V1/orders/'
    . '?searchCriteria[filter_groups][0][filters][0][field]=status&searchCriteria[filter_groups][0][filters][0][value]=pending'
    //. '&searchCriteria[sortOrders][0][field]=increment_id' // To sort Desc
    . '&fields=items[increment_id,entity_id]';

//print file_get_contents($url, false, $context) . "\n"; // Check full json if you want

// Generate json output
$json_contents = file_get_contents($url, false, $context) . "\n";
$order = json_decode($json_contents);
$answer = '{';
if ($order->items) { 
    foreach($order->items as $key => $value) {
        $answer .= '"'. $value->entity_id . '":"' . $value->increment_id . '",';
    }
    $answer = substr($answer,0,strlen($answer)-1);
    $answer .= '}';
    echo $answer;
}

This works for me (I edited my code a little to show):

// First log in
$url = 'http://[your-domain]/rest/V1/integration/admin/token';
$body = '{"username":"user", "password":"pass"}';
$header = 'Content-type: application/json';
$options = [    'method' => 'POST',
                'content' => $body,
                'header' => $header
           ];
$context = stream_context_create(['http' => $options]);
$token = json_decode(file_get_contents($url, false, $context));

// Some settings

$header = 'Authorization: Bearer ' . $token . "\r\n"
    . 'Content-type: application/json';
$options = [    'method' => 'GET',
                'content' => $body,
                'header' => $header
           ];
$context = stream_context_create(['http' => $options]);

$url = 'http://[your-domain]/index.php/rest/V1/orders/'
    . '?searchCriteria[filter_groups][0][filters][0][field]=status&searchCriteria[filter_groups][0][filters][0][value]=pending'
    //. '&searchCriteria[sortOrders][0][field]=increment_id' // To sort Desc
    . '&fields=items[increment_id,entity_id]';

//print file_get_contents($url, false, $context) . "\n"; // Check full json if you want

// Generate json output
$json_contents = file_get_contents($url, false, $context) . "\n";
$order = json_decode($json_contents);
$answer = '{';
if ($order->items) { 
    foreach($order->items as $key => $value) {
        $answer .= '"'. $value->entity_id . '":"' . $value->increment_id . '",';
    }
    $answer = substr($answer,0,strlen($answer)-1);
    $answer .= '}';
    echo $answer;
}