diff -u b/core/modules/filter/migration_templates/d6_filter_format.yml b/core/modules/filter/migration_templates/d6_filter_format.yml
--- b/core/modules/filter/migration_templates/d6_filter_format.yml
+++ b/core/modules/filter/migration_templates/d6_filter_format.yml
@@ -27,6 +27,8 @@
             - filter_url
             - filter_htmlcorrector
             - filter_html_escape
+          php:
+            - filter_null
       settings:
         plugin: filter_settings
         source: settings
only in patch2:
unchanged:
--- a/core/modules/filter/tests/src/Kernel/Migrate/d6/MigrateFilterFormatTest.php
+++ b/core/modules/filter/tests/src/Kernel/Migrate/d6/MigrateFilterFormatTest.php
@@ -3,6 +3,7 @@
 namespace Drupal\Tests\filter\Kernel\Migrate\d6;
 
 use Drupal\filter\Entity\FilterFormat;
+use Drupal\filter\FilterFormatInterface;
 use Drupal\Tests\migrate_drupal\Kernel\d6\MigrateDrupal6TestBase;
 
 /**
@@ -39,14 +40,16 @@ public function testFilterFormat() {
     $this->assertFalse(isset($filters['filter_html_image_secure']));
 
     // Check variables migrated into filter.
-    $this->assertIdentical('<a href hreflang> <em> <strong> <cite> <code> <ul type> <ol start type> <li> <dl> <dt> <dd>', $filters['filter_html']['settings']['allowed_html']);
-    $this->assertIdentical(TRUE, $filters['filter_html']['settings']['filter_html_help']);
-    $this->assertIdentical(FALSE, $filters['filter_html']['settings']['filter_html_nofollow']);
-    $this->assertIdentical(72, $filters['filter_url']['settings']['filter_url_length']);
+    $this->assertSame('<a href hreflang> <em> <strong> <cite> <code> <ul type> <ol start type> <li> <dl> <dt> <dd>', $filters['filter_html']['settings']['allowed_html']);
+    $this->assertSame(TRUE, $filters['filter_html']['settings']['filter_html_help']);
+    $this->assertSame(FALSE, $filters['filter_html']['settings']['filter_html_nofollow']);
+    $this->assertSame(72, $filters['filter_url']['settings']['filter_url_length']);
 
     // Check that the PHP code filter is converted to filter_null.
-    $filters = FilterFormat::load('php_code')->get('filters');
-    $this->assertTrue(isset($filters['filter_null']));
+    /** @var FilterFormatInterface $format */
+    $format = FilterFormat::load('php_code');
+    $this->assertInstanceOf(FilterFormatInterface::class, $format);
+    $this->assertArrayHasKey('filter_null', $format->get('filters'));
   }
 
 }