diff --git a/tests/phpunit/includes/abstract-testcase.php b/tests/phpunit/includes/abstract-testcase.php
index 6a98dbf19d..d9953ec8e2 100644
--- a/tests/phpunit/includes/abstract-testcase.php
+++ b/tests/phpunit/includes/abstract-testcase.php
@@ -1421,35 +1421,51 @@ abstract class WP_UnitTestCase_Base extends PHPUnit_Adapter_TestCase {
 	 * @param string $path Path to the directory to scan.
 	 */
 	public function delete_folders( $path ) {
-		$this->matched_dirs = array();
 		if ( ! is_dir( $path ) ) {
 			return;
 		}
 
-		$this->scandir( $path );
-		foreach ( array_reverse( $this->matched_dirs ) as $dir ) {
+		$matched_dirs = $this->scandir( $path );
+
+		foreach ( array_reverse( $matched_dirs ) as $dir ) {
 			rmdir( $dir );
 		}
+
 		rmdir( $path );
 	}
 
 	/**
-	 * Retrieves all directories contained inside a directory and stores them in the `$matched_dirs` property.
+	 * Retrieves all directories contained inside a directory.
 	 * Hidden directories are ignored.
 	 *
 	 * This is a helper for the `delete_folders()` method.
 	 *
 	 * @since 4.1.0
+	 * @since 6.1.0 No longer sets a (dynamic) property to keep track of the directories,
+	 *              but returns an array of the directories instead.
 	 *
 	 * @param string $dir Path to the directory to scan.
+	 * @return string[] List of directories.
 	 */
 	public function scandir( $dir ) {
+		$matched_dirs = array();
+
 		foreach ( scandir( $dir ) as $path ) {
 			if ( 0 !== strpos( $path, '.' ) && is_dir( $dir . '/' . $path ) ) {
-				$this->matched_dirs[] = $dir . '/' . $path;
-				$this->scandir( $dir . '/' . $path );
+				$matched_dirs[] = array( $dir . '/' . $path );
+				$matched_dirs[] = $this->scandir( $dir . '/' . $path );
 			}
 		}
+
+		/*
+		 * Compatibility check for PHP < 7.4, where array_merge() expects at least one array.
+		 * See: https://3v4l.org/BIQMA
+		 */
+		if ( array() === $matched_dirs ) {
+			return array();
+		}
+
+		return array_merge( ...$matched_dirs );
 	}
 
 	/**