Add function is_local_url() to check if url is local.

include/network.php

@@ -559,6 +559,14 @@ function z_dns_check($h,$check_mx = 0) {
 	return((@dns_get_record($h,$opts) || filter_var($h, FILTER_VALIDATE_IP)) ? true : false);
 }
 
+function is_local_url($url) {
+  if (str_starts_with($url, z_root()) || str_starts_with($url, '/')) {
+    return true;
+  }
+
+  return false;
+}
+
 /**
  * @brief Validates a given URL.
  *

tests/unit/includes/NetworkTest.php

@@ -0,0 +1,33 @@
+<?php
+/**
+ * tests function from include/network.php
+ *
+ * @package test.util
+ */
+
+use PHPUnit\Framework\TestCase;
+
+require_once('include/network.php');
+
+class NetworkTest extends TestCase {
+
+  public function setup() : void {
+    \App::set_baseurl("https://mytest.org");
+  }
+
+  /**
+   * @dataProvider localUrlTestProvider
+   */
+  public function testIsLocalURL($url, $expected) {
+    $this->assertEquals($expected, is_local_url($url));
+  }
+
+  public function localUrlTestProvider() : array {
+    return [
+      [ '/some/path', true ],
+      [ 'https://mytest.org/some/path', true ],
+      [ 'https://other.site/some/path', false ],
+    ];
+  }
+}
+