diff --git a/lib/setuplib.php b/lib/setuplib.php
index 145c4a47fa1..6f7905bff22 100644
--- a/lib/setuplib.php
+++ b/lib/setuplib.php
@@ -1077,7 +1077,10 @@ function workaround_max_input_vars() {
         return;
     }
 
-    if (count($_POST, COUNT_RECURSIVE) < $max) {
+    // Worst case is advanced checkboxes which use up to two max_input_vars
+    // slots for each entry in $_POST, because of sending two fields with the
+    // same name. So count everything twice just in case.
+    if (count($_POST, COUNT_RECURSIVE) * 2 < $max) {
         return;
     }
 
@@ -1093,6 +1096,15 @@ function workaround_max_input_vars() {
     $fun = create_function('$p', 'return implode("'.$delim.'", $p);');
     $chunks = array_map($fun, array_chunk(explode($delim, $str), $max));
 
+    // Clear everything from existing $_POST array, otherwise it might be included
+    // twice (this affects array params primarily).
+    foreach ($_POST as $key => $value) {
+        unset($_POST[$key]);
+        // Also clear from request array - but only the things that are in $_POST,
+        // that way it will leave the things from a get request if any.
+        unset($_REQUEST[$key]);
+    }
+
     foreach ($chunks as $chunk) {
         $values = array();
         parse_str($chunk, $values);
diff --git a/lib/tests/behat/largeforms.feature b/lib/tests/behat/largeforms.feature
new file mode 100644
index 00000000000..f1415ed169b
--- /dev/null
+++ b/lib/tests/behat/largeforms.feature
@@ -0,0 +1,108 @@
+@core
+Feature: Forms with a large number of fields
+  In order to use certain forms on large Moodle installations
+  As an admin
+  I need forms to work with more fields than the PHP max_input_vars setting
+
+  Background:
+    # Get to the fixture page.
+    Given the following "courses" exist:
+      | fullname | shortname | format |
+      | Course 1 | C1        | topics |
+    And the following "activities" exist:
+      | activity   | name | intro                                                                   | course | idnumber |
+      | label      | L1   | <a href="../lib/tests/fixtures/max_input_vars_test.php">FixtureLink</a> | C1     | label1   |
+    When I log in as "admin"
+    And I am on site homepage
+    And I follow "Course 1"
+    And I follow "FixtureLink"
+
+  # Note: These tests do not actually use JavaScript but they don't work with
+  # the headless 'browser'.
+  @javascript
+  Scenario: Small form with checkboxes (not using workaround)
+    When I follow "Advanced checkboxes / Small"
+    And I press "Submit here!"
+    Then I should see "_qf__core_max_input_vars_test_form=1"
+    And I should see "mform_isexpanded_id_general=1"
+    And I should see "arraytest=[13,42]"
+    And I should see "array2test=[13,42]"
+    And I should see "submitbutton=Submit here!"
+    And I should see "Bulk checkbox success: true"
+
+  @javascript
+  Scenario: Medium length form with checkboxes (needs workaround)
+    When I follow "Advanced checkboxes / Below limit"
+    And I press "Submit here!"
+    Then I should see "_qf__core_max_input_vars_test_form=1"
+    And I should see "mform_isexpanded_id_general=1"
+    And I should see "arraytest=[13,42]"
+    And I should see "array2test=[13,42]"
+    And I should see "submitbutton=Submit here!"
+    And I should see "Bulk checkbox success: true"
+
+  @javascript
+  Scenario: Exact PHP limit length form with checkboxes (uses workaround but doesn't need it)
+    When I follow "Advanced checkboxes / Exact PHP limit"
+    And I press "Submit here!"
+    Then I should see "_qf__core_max_input_vars_test_form=1"
+    And I should see "mform_isexpanded_id_general=1"
+    And I should see "arraytest=[13,42]"
+    And I should see "array2test=[13,42]"
+    And I should see "submitbutton=Submit here!"
+    And I should see "Bulk checkbox success: true"
+
+  @javascript
+  Scenario: Longer than the limit with checkboxes (needs workaround)
+    When I follow "Advanced checkboxes / Above limit"
+    And I press "Submit here!"
+    Then I should see "_qf__core_max_input_vars_test_form=1"
+    And I should see "mform_isexpanded_id_general=1"
+    And I should see "arraytest=[13,42]"
+    And I should see "array2test=[13,42]"
+    And I should see "submitbutton=Submit here!"
+    And I should see "Bulk checkbox success: true"
+
+  @javascript
+  Scenario: Small form with array fields (not using workaround)
+    When I follow "Select options / Small"
+    And I press "Submit here!"
+    Then I should see "_qf__core_max_input_vars_test_form=1"
+    And I should see "mform_isexpanded_id_general=1"
+    And I should see "arraytest=[13,42]"
+    And I should see "array2test=[13,42]"
+    And I should see "submitbutton=Submit here!"
+    And I should see "Bulk array success: true"
+
+  @javascript
+  Scenario: Below limit form with array fields (uses workaround but doesn't need it)
+    When I follow "Select options / Below limit"
+    And I press "Submit here!"
+    Then I should see "_qf__core_max_input_vars_test_form=1"
+    And I should see "mform_isexpanded_id_general=1"
+    And I should see "arraytest=[13,42]"
+    And I should see "array2test=[13,42]"
+    And I should see "submitbutton=Submit here!"
+    And I should see "Bulk array success: true"
+
+  @javascript
+  Scenario: Exact PHP limit length form with array fields (uses workaround but doesn't need it)
+    When I follow "Select options / Exact PHP limit"
+    And I press "Submit here!"
+    Then I should see "_qf__core_max_input_vars_test_form=1"
+    And I should see "mform_isexpanded_id_general=1"
+    And I should see "arraytest=[13,42]"
+    And I should see "array2test=[13,42]"
+    And I should see "submitbutton=Submit here!"
+    And I should see "Bulk array success: true"
+
+  @javascript
+  Scenario: Longer than the limit with array fields (needs workaround)
+    When I follow "Select options / Above limit"
+    And I press "Submit here!"
+    Then I should see "_qf__core_max_input_vars_test_form=1"
+    And I should see "mform_isexpanded_id_general=1"
+    And I should see "arraytest=[13,42]"
+    And I should see "array2test=[13,42]"
+    And I should see "submitbutton=Submit here!"
+    And I should see "Bulk array success: true"
diff --git a/lib/tests/fixtures/max_input_vars_test.php b/lib/tests/fixtures/max_input_vars_test.php
new file mode 100644
index 00000000000..8a715a8594f
--- /dev/null
+++ b/lib/tests/fixtures/max_input_vars_test.php
@@ -0,0 +1,231 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Fixture for Behat test of the max_input_vars handling for large forms.
+ *
+ * @package core
+ * @copyright 2015 The Open University
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+require(__DIR__ . '/../../../config.php');
+require_once($CFG->libdir . '/formslib.php');
+
+// Behat test fixture only.
+defined('BEHAT_SITE_RUNNING') || die('Only available on Behat test server');
+
+/**
+ * Form for testing max_input_vars.
+ *
+ * @copyright 2015 The Open University
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class core_max_input_vars_test_form extends moodleform {
+    /**
+     * Form definition.
+     */
+    public function definition() {
+        global $CFG, $PAGE;
+
+        $mform =& $this->_form;
+
+        $mform->addElement('header', 'general', '');
+        $mform->addElement('hidden', 'type', $this->_customdata['type']);
+        $mform->setType('type', PARAM_ALPHA);
+
+        // This is similar to how the selects are created for the role tables,
+        // without using a Moodle form element.
+        $select = html_writer::select(array(13 => 'ArrayOpt13', 42 => 'ArrayOpt4', 666 => 'ArrayOpt666'),
+                'arraytest[]', array(13, 42), false, array('multiple' => 'multiple', 'size' => 10));
+        $mform->addElement('static', 'arraybit', $select);
+
+        switch ($this->_customdata['control']) {
+            case 'c' :
+                // Create a whole stack of checkboxes.
+                for ($i = 0; $i < $this->_customdata['fieldcount']; $i++) {
+                    $mform->addElement('advcheckbox', 'test_c' . $i, 'Checkbox ' . $i);
+                }
+                break;
+
+            case 'a' :
+                // Create a very large array input type field.
+                $options = array();
+                $values = array();
+                for ($i = 0; $i < $this->_customdata['fieldcount']; $i++) {
+                    $options[$i] = 'BigArray ' . $i;
+                    if ($i !== 3) {
+                        $values[] = $i;
+                    }
+                }
+                $select = html_writer::select($options,
+                        'test_a[]', $values, false, array('multiple' => 'multiple', 'size' => 50));
+                $mform->addElement('static', 'bigarraybit', $select);
+                break;
+        }
+
+        // For the sake of it, let's have a second array.
+        $select = html_writer::select(array(13 => 'Array2Opt13', 42 => 'Array2Opt4', 666 => 'Array2Opt666'),
+                'array2test[]', array(13, 42), false, array('multiple' => 'multiple', 'size' => 10));
+        $mform->addElement('static', 'array2bit', $select);
+
+        $mform->addElement('submit', 'submitbutton', 'Submit here!');
+    }
+}
+
+require_login();
+
+$context = context_system::instance();
+
+$type = optional_param('type', '', PARAM_ALPHA);
+
+// Set up the page details.
+$PAGE->set_url(new moodle_url('/lib/tests/fixtures/max_input_vars_test.php'));
+$PAGE->set_context($context);
+
+if ($type) {
+    // Make it work regardless of max_input_vars setting on server, within reason.
+    if ($type[1] === 's') {
+        // Small enough to definitely fit in the area.
+        $fieldcount = 10;
+    } else if ($type[1] === 'm') {
+        // Just under the limit (will go over for advancedcheckbox).
+        $fieldcount = (int)ini_get('max_input_vars') - 100;
+    } else if ($type[1] === 'e') {
+        // Exactly on the PHP limit, taking into account extra form fields
+        // and the double fields for checkboxes.
+        if ($type[0] === 'c') {
+            $fieldcount = (int)ini_get('max_input_vars') / 2 - 2;
+        } else {
+            $fieldcount = (int)ini_get('max_input_vars') - 11;
+        }
+    } else if ($type[1] === 'l') {
+        // Just over the limit.
+        $fieldcount = (int)ini_get('max_input_vars') + 100;
+    }
+
+    $mform = new core_max_input_vars_test_form('max_input_vars_test.php',
+            array('type' => $type, 'fieldcount' => $fieldcount, 'control' => $type[0]));
+    if ($type[0] === 'c') {
+        $data = array();
+        for ($i = 0; $i < $fieldcount; $i++) {
+            if ($i === 3) {
+                // Everything is set except number 3.
+                continue;
+            }
+            $data['test_c' . $i] = 1;
+        }
+        $mform->set_data($data);
+    }
+}
+
+echo $OUTPUT->header();
+
+if ($type && ($result = $mform->get_data())) {
+    $testc = array();
+    $testa = array();
+    foreach ($_POST as $key => $value) {
+        $matches = array();
+        // Handle the 'bulk' ones separately so we can show success/fail rather
+        // than outputting a thousand items; also makes it possible to Behat-test
+        // without depending on specific value of max_input_vars.
+        if (preg_match('~^test_c([0-9]+)$~', $key, $matches)) {
+            $testc[(int)$matches[1]] = $value;
+        } else if ($key === 'test_a') {
+            $testa = $value;
+        } else {
+            // Other fields are output straight off.
+            if (is_array($value)) {
+                echo html_writer::div(s($key) . '=[' . s(implode(',', $value)) . ']');
+            } else {
+                echo html_writer::div(s($key) . '=' . s($value));
+            }
+        }
+    }
+
+    // Confirm that the bulk results are correct.
+    switch ($type[0]) {
+        case 'c' :
+            $success = true;
+            for ($i = 0; $i < $fieldcount; $i++) {
+                if (!array_key_exists($i, $testc)) {
+                    $success = false;
+                    break;
+                }
+                if ($testc[$i] != ($i == 3 ? 0 : 1)) {
+                    $success = false;
+                    break;
+                }
+            }
+            if (array_key_exists($fieldcount, $testc)) {
+                $success = false;
+            }
+            // Check using Moodle form and _param functions too.
+            $key = 'test_c' . ($fieldcount - 1);
+            if (empty($result->{$key})) {
+                $success = false;
+            }
+            if (optional_param($key, 0, PARAM_INT) !== 1) {
+                $success = false;
+            }
+            echo html_writer::div('Bulk checkbox success: ' . ($success ? 'true' : 'false'));
+            break;
+
+        case 'a' :
+            $success = true;
+            for ($i = 0; $i < $fieldcount; $i++) {
+                if ($i === 3) {
+                    if (in_array($i, $testa)) {
+                        $success = false;
+                        break;
+                    }
+                } else {
+                    if (!in_array($i, $testa)) {
+                        $success = false;
+                        break;
+                    }
+                }
+            }
+            if (in_array($fieldcount, $testa)) {
+                $success = false;
+            }
+            // Check using Moodle _param function. The form does not include these
+            // fields so it won't be in the form result.
+            $array = optional_param_array('test_a', array(), PARAM_INT);
+            if ($array != $testa) {
+                $success = false;
+            }
+            echo html_writer::div('Bulk array success: ' . ($success ? 'true' : 'false'));
+            break;
+    }
+
+} else if ($type) {
+    $mform->display();
+}
+
+// Show links to each available type of test.
+echo html_writer::start_tag('ul');
+foreach (array('c' => 'Advanced checkboxes',
+        'a' => 'Select options') as $control => $controlname) {
+    foreach (array('s' => 'Small', 'm' => 'Below limit', 'e' => 'Exact PHP limit',
+            'l' => 'Above limit') as $size => $sizename) {
+        echo html_writer::tag('li', html_writer::link('max_input_vars_test.php?type=' .
+                $control . $size, $controlname . ' / ' . $sizename));
+    }
+}
+echo html_writer::end_tag('ul');
+
+echo $OUTPUT->footer();