diff --git a/admin/tool/brickfield/classes/accessibility.php b/admin/tool/brickfield/classes/accessibility.php
index 877511ff649..35f3510b9e8 100644
--- a/admin/tool/brickfield/classes/accessibility.php
+++ b/admin/tool/brickfield/classes/accessibility.php
@@ -323,7 +323,7 @@ class accessibility {
     /**
      * This function runs one specified check on the html item
      *
-     * @param string $html The html string to be analysed; might be NULL.
+     * @param string|null $html The html string to be analysed; might be NULL.
      * @param int $contentid The content area ID
      * @param int $errid The error ID
      * @param string $check The check name to run
@@ -333,7 +333,7 @@ class accessibility {
      * @throws \dml_exception
      */
     public static function run_one_check(
-        string $html = null,
+        ?string $html,
         int $contentid,
         int $errid,
         string $check,
diff --git a/availability/classes/tree.php b/availability/classes/tree.php
index 6f5d08b3711..36e759e763a 100644
--- a/availability/classes/tree.php
+++ b/availability/classes/tree.php
@@ -482,13 +482,13 @@ class tree extends tree_node {
      *
      * @param bool $not True if there is a NOT in effect
      * @param info $info Information about location of condition tree
-     * @param result $result Result object if this is a student display, else null
+     * @param result|null $result Result object if this is a student display, else null
      * @param bool $root True if this is the root item
      * @param bool $hidden Staff display; true if this tree has show=false (from parent)
      * @return string|renderable Information to render
      */
     protected function get_full_information_recursive(
-            $not, info $info, result $result = null, $root, $hidden = false) {
+            $not, info $info, ?result $result, $root, $hidden = false) {
         // Get list of children - either full list, or those which are shown.
         $children = $this->children;
         $staff = true;
diff --git a/calendar/classes/local/event/entities/event.php b/calendar/classes/local/event/entities/event.php
index b3a7c748768..b0f3f428b14 100644
--- a/calendar/classes/local/event/entities/event.php
+++ b/calendar/classes/local/event/entities/event.php
@@ -118,16 +118,16 @@ class event implements event_interface {
      * @param int                        $id             The event's ID in the database.
      * @param string                     $name           The event's name.
      * @param description_interface      $description    The event's description.
-     * @param proxy_interface            $category       The category associated with the event.
-     * @param proxy_interface            $course         The course associated with the event.
-     * @param proxy_interface            $group          The group associated with the event.
-     * @param proxy_interface            $user           The user associated with the event.
-     * @param event_collection_interface $repeats        Collection of repeat events.
-     * @param proxy_interface            $coursemodule   The course module that created the event.
+     * @param proxy_interface|null       $category       The category associated with the event.
+     * @param proxy_interface|null       $course         The course associated with the event.
+     * @param proxy_interface|null       $group          The group associated with the event.
+     * @param proxy_interface|null       $user           The user associated with the event.
+     * @param event_collection_interface|null $repeats   Collection of repeat events.
+     * @param proxy_interface|null       $coursemodule   The course module that created the event.
      * @param string                     $type           The event's type.
      * @param times_interface            $times          The times associated with the event.
      * @param bool                       $visible        The event's visibility. True for visible, false for invisible.
-     * @param proxy_interface            $subscription   The event's subscription.
+     * @param proxy_interface|null       $subscription   The event's subscription.
      * @param string                     $location       The event's location.
      * @param string                     $component      The event's component.
      */
@@ -135,12 +135,12 @@ class event implements event_interface {
         $id,
         $name,
         description_interface $description,
-        proxy_interface $category = null,
-        proxy_interface $course = null,
-        proxy_interface $group = null,
-        proxy_interface $user = null,
-        event_collection_interface $repeats = null,
-        proxy_interface $coursemodule = null,
+        ?proxy_interface $category,
+        ?proxy_interface $course,
+        ?proxy_interface $group,
+        ?proxy_interface $user,
+        ?event_collection_interface $repeats,
+        ?proxy_interface $coursemodule,
         $type,
         times_interface $times,
         $visible,
diff --git a/contentbank/classes/output/bankcontent.php b/contentbank/classes/output/bankcontent.php
index a595f195c16..a2dfb0e0d20 100644
--- a/contentbank/classes/output/bankcontent.php
+++ b/contentbank/classes/output/bankcontent.php
@@ -69,10 +69,10 @@ class bankcontent implements renderable, templatable {
      *
      * @param \core_contentbank\content[] $contents   Array of content bank contents.
      * @param array $toolbar List of content bank toolbar options.
-     * @param \context $context Optional context to check (default null)
+     * @param \context|null $context Optional context to check (default null)
      * @param contentbank $cb Contenbank object.
      */
-    public function __construct(array $contents, array $toolbar, \context $context = null, contentbank $cb) {
+    public function __construct(array $contents, array $toolbar, ?\context $context, contentbank $cb) {
         $this->contents = $contents;
         $this->toolbar = $toolbar;
         $this->context = $context;
diff --git a/lib/dml/moodle_database.php b/lib/dml/moodle_database.php
index f9f7aada275..8d1f0351056 100644
--- a/lib/dml/moodle_database.php
+++ b/lib/dml/moodle_database.php
@@ -415,12 +415,12 @@ abstract class moodle_database {
     /**
      * This should be called before each db query.
      * @param string $sql The query string.
-     * @param array $params An array of parameters.
+     * @param array|null $params An array of parameters.
      * @param int $type The type of query. ( SQL_QUERY_SELECT | SQL_QUERY_AUX | SQL_QUERY_INSERT | SQL_QUERY_UPDATE | SQL_QUERY_STRUCTURE )
      * @param mixed $extrainfo This is here for any driver specific extra information.
      * @return void
      */
-    protected function query_start($sql, array $params=null, $type, $extrainfo=null) {
+    protected function query_start($sql, ?array $params, $type, $extrainfo=null) {
         if ($this->loggingquery) {
             return;
         }
diff --git a/lib/dml/moodle_read_slave_trait.php b/lib/dml/moodle_read_slave_trait.php
index 7f06e5efb8d..7a2da7d134c 100644
--- a/lib/dml/moodle_read_slave_trait.php
+++ b/lib/dml/moodle_read_slave_trait.php
@@ -262,12 +262,12 @@ trait moodle_read_slave_trait {
     /**
      * Called before each db query.
      * @param string $sql
-     * @param array $params array of parameters
+     * @param array|null $params An array of parameters.
      * @param int $type type of query
      * @param mixed $extrainfo driver specific extra information
      * @return void
      */
-    protected function query_start($sql, array $params = null, $type, $extrainfo = null) {
+    protected function query_start($sql, ?array $params, $type, $extrainfo = null) {
         parent::query_start($sql, $params, $type, $extrainfo);
         $this->select_db_handle($type, $sql);
     }
diff --git a/lib/dml/oci_native_moodle_database.php b/lib/dml/oci_native_moodle_database.php
index 06e6b3a8d45..22816826f91 100644
--- a/lib/dml/oci_native_moodle_database.php
+++ b/lib/dml/oci_native_moodle_database.php
@@ -245,12 +245,12 @@ class oci_native_moodle_database extends moodle_database {
     /**
      * Called before each db query.
      * @param string $sql
-     * @param array array of parameters
+     * @param array|null $params An array of parameters.
      * @param int $type type of query
      * @param mixed $extrainfo driver specific extra information
      * @return void
      */
-    protected function query_start($sql, array $params=null, $type, $extrainfo=null) {
+    protected function query_start($sql, ?array $params, $type, $extrainfo=null) {
         parent::query_start($sql, $params, $type, $extrainfo);
         // oci driver tents to send debug to output, we do not need that ;-)
         $this->last_error_reporting = error_reporting(0);
diff --git a/lib/dml/pdo_moodle_database.php b/lib/dml/pdo_moodle_database.php
index 2a0989fd583..ba20e29de12 100644
--- a/lib/dml/pdo_moodle_database.php
+++ b/lib/dml/pdo_moodle_database.php
@@ -614,12 +614,12 @@ abstract class pdo_moodle_database extends moodle_database {
      * Overridden to ensure $this->lastErorr is reset each query
      *
      * @param string $sql
-     * @param array array of parameters
+     * @param array|null $params An array of parameters.
      * @param int $type type of query
      * @param mixed $extrainfo driver specific extra information
      * @return void
      */
-    protected function query_start($sql, array $params=null, $type, $extrainfo=null) {
+    protected function query_start($sql, ?array $params, $type, $extrainfo=null) {
         $this->lastError = null;
         parent::query_start($sql, $params, $type, $extrainfo);
     }
diff --git a/lib/dml/pgsql_native_moodle_database.php b/lib/dml/pgsql_native_moodle_database.php
index ba0abe1d1b1..8fde868843a 100644
--- a/lib/dml/pgsql_native_moodle_database.php
+++ b/lib/dml/pgsql_native_moodle_database.php
@@ -315,12 +315,12 @@ class pgsql_native_moodle_database extends moodle_database {
     /**
      * Called before each db query.
      * @param string $sql
-     * @param array array of parameters
+     * @param array|null $params An array of parameters.
      * @param int $type type of query
      * @param mixed $extrainfo driver specific extra information
      * @return void
      */
-    protected function query_start($sql, array $params=null, $type, $extrainfo=null) {
+    protected function query_start($sql, ?array $params, $type, $extrainfo=null) {
         $this->read_slave_query_start($sql, $params, $type, $extrainfo);
         // pgsql driver tends to send debug to output, we do not need that.
         $this->last_error_reporting = error_reporting(0);
diff --git a/lib/dml/sqlsrv_native_moodle_database.php b/lib/dml/sqlsrv_native_moodle_database.php
index 55ad90ab49a..37a1d9fe4ba 100644
--- a/lib/dml/sqlsrv_native_moodle_database.php
+++ b/lib/dml/sqlsrv_native_moodle_database.php
@@ -306,12 +306,12 @@ class sqlsrv_native_moodle_database extends moodle_database {
     /**
      * Called before each db query.
      * @param string $sql
-     * @param array $params array of parameters
+     * @param array|null $params An array of parameters.
      * @param int $type type of query
      * @param mixed $extrainfo driver specific extra information
      * @return void
      */
-    protected function query_start($sql, array $params = null, $type, $extrainfo = null) {
+    protected function query_start($sql, ?array $params, $type, $extrainfo = null) {
         parent::query_start($sql, $params, $type, $extrainfo);
     }
 
diff --git a/lib/dml/tests/fixtures/read_slave_moodle_database.php b/lib/dml/tests/fixtures/read_slave_moodle_database.php
index 7659854c7b3..ecf5d572b7c 100644
--- a/lib/dml/tests/fixtures/read_slave_moodle_database.php
+++ b/lib/dml/tests/fixtures/read_slave_moodle_database.php
@@ -91,12 +91,12 @@ class read_slave_moodle_database extends test_moodle_database {
     /**
      * Query wrapper that calls query_start() and query_end()
      * @param string $sql
-     * @param array $params
+     * @param array|null $params
      * @param int $querytype
      * @param ?callable $callback
      * @return string $handle handle property
      */
-    public function with_query_start_end($sql, array $params = null, $querytype, $callback = null) {
+    public function with_query_start_end($sql, ?array $params, $querytype, $callback = null) {
         $this->query_start($sql, $params, $querytype);
         $ret = $this->handle;
         if ($callback) {
diff --git a/lib/dml/tests/fixtures/test_moodle_read_slave_trait.php b/lib/dml/tests/fixtures/test_moodle_read_slave_trait.php
index 6172c3c174e..1d9d6639156 100644
--- a/lib/dml/tests/fixtures/test_moodle_read_slave_trait.php
+++ b/lib/dml/tests/fixtures/test_moodle_read_slave_trait.php
@@ -95,11 +95,11 @@ trait test_moodle_read_slave_trait {
     /**
      * Upgrade to public
      * @param string $sql
-     * @param array $params
+     * @param array|null $params
      * @param int $type
      * @param array $extrainfo
      */
-    public function query_start($sql, array $params = null, $type, $extrainfo = null) {
+    public function query_start($sql, ?array $params, $type, $extrainfo = null) {
         return parent::query_start($sql, $params, $type);
     }
 
diff --git a/lib/filestorage/tgz_packer.php b/lib/filestorage/tgz_packer.php
index 23a8cd3c64e..2d67ade7b10 100644
--- a/lib/filestorage/tgz_packer.php
+++ b/lib/filestorage/tgz_packer.php
@@ -544,12 +544,12 @@ class tgz_packer extends file_packer {
      * @param array $expandedfiles List of all files to archive (output)
      * @param string $archivepath Current path within archive
      * @param string $path OS path on disk
-     * @param file_progress $progress Progress indicator or null if none
+     * @param file_progress|null $progress Progress indicator or null if none
      * @param int $done Value for progress indicator
      * @return bool True if successful
      */
     protected function list_files_path(array &$expandedfiles, $archivepath, $path,
-            file_progress $progress = null, $done) {
+            ?file_progress $progress , $done) {
         if (is_dir($path)) {
             // Unless we're using this directory as archive root, add a
             // directory entry.
diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index d555ca6f7a8..5f57ce8869b 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -786,13 +786,13 @@ function validate_param($param, $type, $allownull=NULL_NOT_ALLOWED, $debuginfo='
  * $options = clean_param($options, PARAM_INT);
  * </code>
  *
- * @param array $param the variable array we are cleaning
+ * @param array|null $param the variable array we are cleaning
  * @param string $type expected format of param after cleaning.
  * @param bool $recursive clean recursive arrays
  * @return array
  * @throws coding_exception
  */
-function clean_param_array(array $param = null, $type, $recursive = false) {
+function clean_param_array(?array $param, $type, $recursive = false) {
     // Convert null to empty array.
     $param = (array)$param;
     foreach ($param as $key => $value) {
diff --git a/lib/outputcomponents.php b/lib/outputcomponents.php
index f1b6caa0d5a..925f2f49b0b 100644
--- a/lib/outputcomponents.php
+++ b/lib/outputcomponents.php
@@ -4742,12 +4742,12 @@ class action_menu_link extends action_link implements renderable {
      * Constructs the object.
      *
      * @param moodle_url $url The URL for the action.
-     * @param pix_icon $icon The icon to represent the action.
+     * @param pix_icon|null $icon The icon to represent the action.
      * @param string $text The text to represent the action.
      * @param bool $primary Whether this is a primary action or not.
      * @param array $attributes Any attribtues associated with the action.
      */
-    public function __construct(moodle_url $url, pix_icon $icon = null, $text, $primary = true, array $attributes = array()) {
+    public function __construct(moodle_url $url, ?pix_icon $icon, $text, $primary = true, array $attributes = array()) {
         parent::__construct($url, $text, null, $attributes, $icon);
         $this->primary = (bool)$primary;
         $this->add_class('menu-action');
@@ -4813,11 +4813,11 @@ class action_menu_link_primary extends action_menu_link {
      * Constructs the object.
      *
      * @param moodle_url $url
-     * @param pix_icon $icon
+     * @param pix_icon|null $icon
      * @param string $text
      * @param array $attributes
      */
-    public function __construct(moodle_url $url, pix_icon $icon = null, $text, array $attributes = array()) {
+    public function __construct(moodle_url $url, ?pix_icon $icon, $text, array $attributes = array()) {
         parent::__construct($url, $icon, $text, true, $attributes);
     }
 }
@@ -4835,11 +4835,11 @@ class action_menu_link_secondary extends action_menu_link {
      * Constructs the object.
      *
      * @param moodle_url $url
-     * @param pix_icon $icon
+     * @param pix_icon|null $icon
      * @param string $text
      * @param array $attributes
      */
-    public function __construct(moodle_url $url, pix_icon $icon = null, $text, array $attributes = array()) {
+    public function __construct(moodle_url $url, ?pix_icon $icon, $text, array $attributes = array()) {
         parent::__construct($url, $icon, $text, false, $attributes);
     }
 }
diff --git a/message/classes/search/base_message.php b/message/classes/search/base_message.php
index a4aa6fe125d..bcb7dd882bf 100644
--- a/message/classes/search/base_message.php
+++ b/message/classes/search/base_message.php
@@ -141,7 +141,7 @@ abstract class base_message extends \core_search\base {
      * @return \moodle_recordset|null Recordset or null if no results possible
      * @throws \coding_exception If context invalid
      */
-    protected function get_document_recordset_helper($modifiedfrom, \context $context = null,
+    protected function get_document_recordset_helper($modifiedfrom, ?\context $context,
             $userfield) {
         global $DB;
 
diff --git a/search/classes/base.php b/search/classes/base.php
index 78c1fe25186..c3678125ff1 100644
--- a/search/classes/base.php
+++ b/search/classes/base.php
@@ -452,7 +452,7 @@ abstract class base {
      * @return array Array with SQL and parameters; both null if no need to query
      * @throws \coding_exception If called with invalid params
      */
-    protected function get_course_level_context_restriction_sql(\context $context = null,
+    protected function get_course_level_context_restriction_sql(?\context $context,
             $coursetable, $paramtype = SQL_PARAMS_QM) {
         global $DB;
 
diff --git a/search/classes/base_mod.php b/search/classes/base_mod.php
index 6015f86d3e5..2f638b849dc 100644
--- a/search/classes/base_mod.php
+++ b/search/classes/base_mod.php
@@ -109,7 +109,7 @@ abstract class base_mod extends base {
      * @return array Array with SQL and parameters; both null if no need to query
      * @throws \coding_exception If called with invalid params
      */
-    protected function get_context_restriction_sql(\context $context = null, $modname, $modtable,
+    protected function get_context_restriction_sql(?\context $context, $modname, $modtable,
             $paramtype = SQL_PARAMS_QM) {
         global $DB;