diff --git a/src/WP_Export_Oxymel.php b/src/WP_Export_Oxymel.php
index 0d3c75e3..5c5b7804 100644
--- a/src/WP_Export_Oxymel.php
+++ b/src/WP_Export_Oxymel.php
@@ -16,8 +16,15 @@ public function optional_cdata( $tag_name, $contents ) {
 	}
 
 	public function cdata( $text ) {
-		if ( is_string( $text ) && ! seems_utf8( $text ) ) {
-			$text = mb_convert_encoding( $text, 'UTF-8' );
+		if ( is_string( $text ) ) {
+			if ( function_exists( 'wp_is_valid_utf8' ) ) {
+				if ( ! wp_is_valid_utf8( $text ) ) {
+					$text = mb_convert_encoding( $text, 'UTF-8' );
+				}
+			} elseif ( ! seems_utf8( $text ) ) { // phpcs:ignore WordPress.WP.DeprecatedFunctions.seems_utf8Found
+				$text = mb_convert_encoding( $text, 'UTF-8' );
+
+			}
 		}
 		return parent::cdata( $text );
 	}
diff --git a/src/WP_Export_Query.php b/src/WP_Export_Query.php
index 4ddc9656..b0df9ee8 100644
--- a/src/WP_Export_Query.php
+++ b/src/WP_Export_Query.php
@@ -245,7 +245,7 @@ private function author_where() {
 
 	private function start_date_where() {
 		global $wpdb;
-		$timestamp = strtotime( $this->filters['start_date'] );
+		$timestamp = $this->filters['start_date'] ? strtotime( $this->filters['start_date'] ) : null;
 		if ( ! $timestamp ) {
 			return;
 		}
@@ -254,6 +254,9 @@ private function start_date_where() {
 
 	private function end_date_where() {
 		global $wpdb;
+		if ( ! $this->filters['end_date'] ) {
+			return;
+		}
 		if ( preg_match( '/^\d{4}-\d{2}$/', $this->filters['end_date'] ) ) {
 			$timestamp = $this->get_timestamp_for_the_last_day_of_a_month( $this->filters['end_date'] );
 		} else {
diff --git a/src/WP_Export_WXR_Formatter.php b/src/WP_Export_WXR_Formatter.php
index 3910cc8e..fe6a29fb 100644
--- a/src/WP_Export_WXR_Formatter.php
+++ b/src/WP_Export_WXR_Formatter.php
@@ -12,6 +12,9 @@
  * Responsible for formatting the data in WP_Export_Query to WXR
  */
 class WP_Export_WXR_Formatter {
+	private $export;
+	private $wxr_version;
+
 	public function __construct( $export ) {
 		$this->export      = $export;
 		$this->wxr_version = WXR_VERSION;
@@ -51,7 +54,7 @@ public function after_posts() {
 	public function header() {
 		$oxymel           = new Oxymel();
 		$wp_generator_tag = $this->export->wp_generator_tag();
-		$comment          = <<<COMMENT
+		$comment          = <<<'COMMENT'
 
  This is a WordPress eXtended RSS file generated by WordPress as an export of your site.
  It contains information about your site's posts, pages, comments, categories, and other content.
diff --git a/src/WP_Map_Iterator.php b/src/WP_Map_Iterator.php
index 9d2d0b3e..a359ba00 100644
--- a/src/WP_Map_Iterator.php
+++ b/src/WP_Map_Iterator.php
@@ -1,6 +1,8 @@
 <?php
 
 class WP_Map_Iterator extends IteratorIterator {
+	private $callback;
+
 	public function __construct( $iterator, $callback ) {
 		$this->callback = $callback;
 		parent::__construct( $iterator );
diff --git a/src/WP_Post_IDs_Iterator.php b/src/WP_Post_IDs_Iterator.php
index b05c6f12..054a76ab 100644
--- a/src/WP_Post_IDs_Iterator.php
+++ b/src/WP_Post_IDs_Iterator.php
@@ -4,7 +4,10 @@ class WP_Post_IDs_Iterator implements Iterator {
 	private $limit = 100;
 	private $post_ids;
 	private $ids_left;
-	private $results = array();
+	private $results          = array();
+	private $global_index     = 0;
+	private $index_in_results = 0;
+	private $db;
 
 	public function __construct( $post_ids, $limit = null ) {
 		$this->db       = $GLOBALS['wpdb'];
@@ -15,19 +18,23 @@ public function __construct( $post_ids, $limit = null ) {
 		}
 	}
 
+	#[\ReturnTypeWillChange]
 	public function current() {
 		return $this->results[ $this->index_in_results ];
 	}
 
+	#[\ReturnTypeWillChange]
 	public function key() {
 		return $this->global_index;
 	}
 
+	#[\ReturnTypeWillChange]
 	public function next() {
 		++$this->index_in_results;
 		++$this->global_index;
 	}
 
+	#[\ReturnTypeWillChange]
 	public function rewind() {
 		$this->results          = array();
 		$this->global_index     = 0;
@@ -35,6 +42,7 @@ public function rewind() {
 		$this->ids_left         = $this->post_ids;
 	}
 
+	#[\ReturnTypeWillChange]
 	public function valid() {
 		if ( isset( $this->results[ $this->index_in_results ] ) ) {
 			return true;