diff --git a/lib/datalib.php b/lib/datalib.php
index 4e47268c417..454bc7ec7b8 100644
--- a/lib/datalib.php
+++ b/lib/datalib.php
@@ -55,14 +55,17 @@ function get_admin() {
     global $CFG, $DB;
 
     static $mainadmin = null;
+    static $prevadmins = null;
 
-    if (isset($mainadmin)) {
+    if (empty($CFG->siteadmins)) {  // Should not happen on an ordinary site.
+        return false;
+    }
+
+    if (isset($mainadmin) and $prevadmins === $CFG->siteadmins) {
         return clone($mainadmin);
     }
 
-    if (empty($CFG->siteadmins)) {  // Should not happen on an ordinary site
-        return false;
-    }
+    $mainadmin = null;
 
     foreach (explode(',', $CFG->siteadmins) as $id) {
         if ($user = $DB->get_record('user', array('id'=>$id, 'deleted'=>0))) {
@@ -72,6 +75,7 @@ function get_admin() {
     }
 
     if ($mainadmin) {
+        $prevadmins = $CFG->siteadmins;
         return clone($mainadmin);
     } else {
         // this should not happen
@@ -95,7 +99,19 @@ function get_admins() {
               FROM {user} u
              WHERE u.deleted = 0 AND u.id IN ($CFG->siteadmins)";
 
-    return $DB->get_records_sql($sql);
+    // We want the same order as in $CFG->siteadmins.
+    $records = $DB->get_records_sql($sql);
+    $admins = array();
+    foreach (explode(',', $CFG->siteadmins) as $id) {
+        $id = (int)$id;
+        if (!isset($records[$id])) {
+            // User does not exist, this should not happen.
+            continue;
+        }
+        $admins[$records[$id]->id] = $records[$id];
+    }
+
+    return $admins;
 }
 
 /**
diff --git a/lib/tests/datalib_test.php b/lib/tests/datalib_test.php
index 2d88ea9d32a..e758c74f857 100644
--- a/lib/tests/datalib_test.php
+++ b/lib/tests/datalib_test.php
@@ -87,4 +87,69 @@ class datalib_testcase extends advanced_testcase {
         $this->assertEquals(array('usersortexact1' => 'search', 'usersortexact2' => 'search',
                 'usersortexact3' => 'search', 'usersortexact4' => 'search', 'usersortexact5' => 'search'), $params);
     }
+
+    public function test_get_admin() {
+        global $CFG, $DB;
+
+        $this->resetAfterTest();
+
+        $this->assertSame('2', $CFG->siteadmins); // Admin always has id 2 in new installs.
+        $defaultadmin = get_admin();
+        $this->assertEquals($defaultadmin->id, 2);
+
+        unset_config('siteadmins');
+        $this->assertFalse(get_admin());
+
+        set_config('siteadmins', -1);
+        $this->assertFalse(get_admin());
+
+        $user1 = $this->getDataGenerator()->create_user();
+        $user2 = $this->getDataGenerator()->create_user();
+
+        set_config('siteadmins', $user1->id.','.$user2->id);
+        $admin = get_admin();
+        $this->assertEquals($user1->id, $admin->id);
+
+        set_config('siteadmins', '-1,'.$user2->id.','.$user1->id);
+        $admin = get_admin();
+        $this->assertEquals($user2->id, $admin->id);
+
+        $odlread = $DB->perf_get_reads();
+        get_admin(); // No DB queries on repeated call expected.
+        get_admin();
+        get_admin();
+        $this->assertEquals($odlread, $DB->perf_get_reads());
+    }
+
+    public function test_get_admins() {
+        global $CFG, $DB;
+
+        $this->resetAfterTest();
+
+        $this->assertSame('2', $CFG->siteadmins); // Admin always has id 2 in new installs.
+
+        $user1 = $this->getDataGenerator()->create_user();
+        $user2 = $this->getDataGenerator()->create_user();
+        $user3 = $this->getDataGenerator()->create_user();
+        $user4 = $this->getDataGenerator()->create_user();
+
+        $admins = get_admins();
+        $this->assertEquals(1, count($admins));
+        $admin = reset($admins);
+        $this->assertTrue(isset($admins[$admin->id]));
+        $this->assertEquals(2, $admin->id);
+
+        unset_config('siteadmins');
+        $this->assertSame(array(), get_admins());
+
+        set_config('siteadmins', -1);
+        $this->assertSame(array(), get_admins());
+
+        set_config('siteadmins', '-1,'.$user2->id.','.$user1->id.','.$user3->id);
+        $this->assertEquals(array($user2->id=>$user2, $user1->id=>$user1, $user3->id=>$user3), get_admins());
+
+        $odlread = $DB->perf_get_reads();
+        get_admins(); // This should make just one query.
+        $this->assertEquals($odlread+1, $DB->perf_get_reads());
+    }
 }