Fix potential fatal error on Interactivity API processing directives

src/wp-includes/interactivity-api/class-wp-interactivity-api.php

@@ -520,7 +520,12 @@ private function _process_directives( string $html ) {
 					array_pop( $tag_stack );
 				}
 			} else {
-				if ( 0 !== count( $p->get_attribute_names_with_prefix( 'data-wp-each-child' ) ) ) {
+				$each_child_attrs = $p->get_attribute_names_with_prefix( 'data-wp-each-child' );
+				if ( null === $each_child_attrs ) {
+					continue;
+				}
+
+				if ( null !== $each_child_attrs && 0 !== count( $each_child_attrs ) ) {
 					/*
 					 * If the tag has a `data-wp-each-child` directive, jump to its closer
 					 * tag because those tags have already been processed.

tests/phpunit/tests/interactivity-api/wpInteractivityAPIDirectivesProcessor.php

@@ -12,6 +12,34 @@
  * @coversDefaultClass WP_Interactivity_API_Directives_Processor
  */
 class Tests_Interactivity_API_WpInteractivityAPIDirectivesProcessor extends WP_UnitTestCase {
+
+	/**
+	 * Regression: _process_directives should not fatal when encountering malformed markup
+	 * (e.g., a stray </br>) that causes get_attribute_names_with_prefix() to return null.
+	 *
+	 * Before the fix: PHP 8+ threw a TypeError from count(null).
+	 */
+	public function test_process_directives_handles_noncountable_each_child_attrs() {
+		$html = '<div><br></br><span data-wp-bind--text="state.foo">x</span></div>';
+
+		$api = new WP_Interactivity_API();
+
+		$ref    = new ReflectionClass( $api );
+		$method = $ref->getMethod( '_process_directives' );
+		$method->setAccessible( true );
+
+		// This would throw if there is an error.
+		$processed = $method->invoke( $api, $html );
+
+		$this->assertIsString( $processed );
+		$this->assertStringContainsString( '<span', $processed );
+		$this->assertStringContainsString( 'data-wp-bind--text', $processed );
+
+		// Assert content still intact despite the malformed </br>.
+		$this->assertStringContainsString( '<div>', $processed );
+	}
+
+
 	/**
 	 * Tests the `get_content_between_balanced_template_tags` method on template
 	 * tags.