nextcloud/settings/ajax/changedisplayname.php

<?php
// Check if we are a user
OCP\JSON::callCheck();
OC_JSON::checkLoggedIn();

$username = isset($_POST["username"]) ? $_POST["username"] : OC_User::getUser();
$displayName = $_POST["displayName"];

$userstatus = null;
if(OC_User::isAdminUser(OC_User::getUser())) {
	$userstatus = 'admin';
}
if(OC_SubAdmin::isUserAccessible(OC_User::getUser(), $username)) {
	$userstatus = 'subadmin';
}

if(is_null($userstatus)) {
	OC_JSON::error( array( "data" => array( "message" => "Authentication error" )));
	exit();
}

// Return Success story
if( OC_User::setDisplayName( $username, $displayName )) {
	OC_JSON::success(array("data" => array( "username" => $username )));
}
else{
	OC_JSON::error(array("data" => array( "message" => "Unable to change display name" )));
}