diff --git a/NEWS b/NEWS
index 18851577a35..0bfaf9b2872 100644
--- a/NEWS
+++ b/NEWS
@@ -6,6 +6,10 @@ PHP                                                                        NEWS
   . Fixed bug GH-21052 (Preloaded constant erroneously propagated to file-cached
     script). (ilutov)
 
+- Standard:
+  . Fixed bug GH-20906 (Assertion failure when messing up output buffers).
+    (ndossche)
+
 12 Mar 2026, PHP 8.5.4
 
 - Core:
diff --git a/ext/standard/basic_functions.c b/ext/standard/basic_functions.c
index 8b72ffffc83..f30db2ce3d6 100644
--- a/ext/standard/basic_functions.c
+++ b/ext/standard/basic_functions.c
@@ -1751,7 +1751,10 @@ PHP_FUNCTION(highlight_file)
 	}
 
 	if (i) {
-		php_output_start_default();
+		if (UNEXPECTED(php_output_start_default() != SUCCESS)) {
+			zend_throw_error(NULL, "Unable to start output handler");
+			RETURN_THROWS();
+		}
 	}
 
 	php_get_highlight_struct(&syntax_highlighter_ini);
@@ -1786,7 +1789,10 @@ PHP_FUNCTION(php_strip_whitespace)
 		Z_PARAM_PATH_STR(filename)
 	ZEND_PARSE_PARAMETERS_END();
 
-	php_output_start_default();
+	if (UNEXPECTED(php_output_start_default() != SUCCESS)) {
+		zend_throw_error(NULL, "Unable to start output handler");
+		RETURN_THROWS();
+	}
 
 	zend_stream_init_filename_ex(&file_handle, filename);
 	zend_save_lexical_state(&original_lex_state);
@@ -1823,7 +1829,10 @@ PHP_FUNCTION(highlight_string)
 	ZEND_PARSE_PARAMETERS_END();
 
 	if (i) {
-		php_output_start_default();
+		if (UNEXPECTED(php_output_start_default() != SUCCESS)) {
+			zend_throw_error(NULL, "Unable to start output handler");
+			RETURN_THROWS();
+		}
 	}
 
 	EG(error_reporting) = E_ERROR;
diff --git a/ext/standard/tests/strings/gh20906_1.phpt b/ext/standard/tests/strings/gh20906_1.phpt
new file mode 100644
index 00000000000..ccb0dfbee56
--- /dev/null
+++ b/ext/standard/tests/strings/gh20906_1.phpt
@@ -0,0 +1,25 @@
+--TEST--
+GH-20906 (Assertion failure when messing up output buffers) - php_strip_whitespace
+--CREDITS--
+vi3tL0u1s
+--FILE--
+<?php
+class A {
+    function __destruct() {
+        php_strip_whitespace(__FILE__);
+        echo "x";
+        $c = new A;
+        ob_start(function () use ($c) { return '/'; }, 1);
+        ob_start(function () use (&$c) { $c = new A; return '/'; }, 1);
+    }
+}
+
+try {
+  new A;
+} catch (Throwable $e) {
+  echo $e::class, ": ", $e->getMessage(), "\n";
+}
+?>
+--EXPECTF--
+%a
+Fatal error: php_strip_whitespace(): Cannot use output buffering in output buffering display handlers in %s on line %d
diff --git a/ext/standard/tests/strings/gh20906_2.phpt b/ext/standard/tests/strings/gh20906_2.phpt
new file mode 100644
index 00000000000..b3ea5cf6ef7
--- /dev/null
+++ b/ext/standard/tests/strings/gh20906_2.phpt
@@ -0,0 +1,21 @@
+--TEST--
+GH-20906 (Assertion failure when messing up output buffers) - highlight_file
+--CREDITS--
+vi3tL0u1s
+--FILE--
+<?php
+class A {
+    function __destruct() {
+        highlight_file(__FILE__, true);
+        echo "x";
+        $c = new A;
+        ob_start(function () use ($c) { return '/'; }, 1);
+        ob_start(function () use (&$c) { $c = new A; return '/'; }, 1);
+    }
+}
+
+new A;
+?>
+--EXPECTF--
+%a
+Fatal error: highlight_file(): Cannot use output buffering in output buffering display handlers in %s on line %d
diff --git a/ext/standard/tests/strings/gh20906_3.phpt b/ext/standard/tests/strings/gh20906_3.phpt
new file mode 100644
index 00000000000..a26fc61bed6
--- /dev/null
+++ b/ext/standard/tests/strings/gh20906_3.phpt
@@ -0,0 +1,21 @@
+--TEST--
+GH-20906 (Assertion failure when messing up output buffers) - highlight_string
+--CREDITS--
+vi3tL0u1s
+--FILE--
+<?php
+class A {
+    function __destruct() {
+        highlight_string(__FILE__, true);
+        echo "x";
+        $c = new A;
+        ob_start(function () use ($c) { return '/'; }, 1);
+        ob_start(function () use (&$c) { $c = new A; return '/'; }, 1);
+    }
+}
+
+new A;
+?>
+--EXPECTF--
+%a
+Fatal error: highlight_string(): Cannot use output buffering in output buffering display handlers in %s on line %d