diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index 3634190663b67..5af0a4a6b32d1 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -5419,7 +5419,25 @@ function wp_widgets_add_menu() {
 function wp_ob_end_flush_all() {
 	$levels = ob_get_level();
 	for ( $i = 0; $i < $levels; $i++ ) {
-		ob_end_flush();
+		$ob_status = ob_get_status();
+
+		// can be not set in some installations
+		if ( ! isset( $ob_status['flags'] ) ) {
+			break;
+		}
+
+		if ( ( $ob_status['flags'] & PHP_OUTPUT_HANDLER_REMOVABLE ) === PHP_OUTPUT_HANDLER_REMOVABLE ) {
+			ob_end_flush();
+			continue;
+		}
+
+		if ( ( $ob_status['flags'] & PHP_OUTPUT_HANDLER_FLUSHABLE ) === PHP_OUTPUT_HANDLER_FLUSHABLE ) {
+			// we can't end it, but at least flush it
+			ob_flush();
+		}
+
+		// can't end any parents either
+		break;
 	}
 }
 
diff --git a/tests/phpunit/tests/functions/wpObEndFlushAll.php b/tests/phpunit/tests/functions/wpObEndFlushAll.php
new file mode 100644
index 0000000000000..dd0746a5ec32a
--- /dev/null
+++ b/tests/phpunit/tests/functions/wpObEndFlushAll.php
@@ -0,0 +1,61 @@
+<?php
+
+/**
+ * @group functions
+ * @group query
+ *
+ * @covers ::wp_ob_end_flush_all
+ */
+class Tests_Functions_wpObEndFlushAll extends WP_UnitTestCase {
+	/**
+	 * Output buffer callback.
+	 *
+	 * @param string $output output buffer
+	 * @return string
+	 */
+	private function my_ob_cb( $output ) {
+		return $output;
+	}
+
+	/**
+	 * @ticket 22239
+	 *
+	 * Must run in a separate process since the output buffer is not endable and would spill to subsequent tests.
+	 * @runInSeparateProcess
+	 * @preserveGlobalState disabled
+	 */
+	public function test_wp_ob_end_flush_all_non_removable() {
+		$this->markTestSkipped(
+			'Non-removable output buffers cannot be tested at the moment, see https://github.com/sebastianbergmann/phpunit/issues/5851.'
+		);
+
+		// Clear any previous errors that could lead to false failures.
+		error_clear_last();
+
+		ob_start( array( $this, 'my_ob_cb' ), 0, 0 );
+
+		wp_ob_end_flush_all();
+
+		$this->assertNull( error_get_last() );
+	}
+
+	public function test_wp_ob_end_flush_all() {
+		$this->expectOutputString( '' );
+
+		// Clear any previous errors that could lead to false failures.
+		error_clear_last();
+
+		ob_start( array( $this, 'my_ob_cb' ) );
+
+		// will not be in phpunit, since we end all buffers below
+		// phpunit's behavior will change once the issue above is fixed though, in which case we would expect "hello" instead of empty string
+		echo 'hello';
+
+		wp_ob_end_flush_all();
+
+		$this->assertNull( error_get_last() );
+
+		// restart phpunit's output buffer
+		ob_start();
+	}
+}