diff --git a/NEWS b/NEWS
index afedd03b153..aaf503112bc 100644
--- a/NEWS
+++ b/NEWS
@@ -34,6 +34,10 @@ PHP                                                                        NEWS
   . Fixed bug GH-16361 (mb_substr overflow on start/length arguments).
     (David Carlier)
 
+- Opcache:
+  . Fixed bug GH-16408 (Array to string conversion warning emitted in
+    optimizer). (ilutov)
+
 - OpenSSL:
   . Fixed bug GH-16357 (openssl may modify member types of certificate arrays).
     (cmb)
diff --git a/Zend/Optimizer/pass1.c b/Zend/Optimizer/pass1.c
index 4c8a03b4a40..9d1b52e428b 100644
--- a/Zend/Optimizer/pass1.c
+++ b/Zend/Optimizer/pass1.c
@@ -34,6 +34,12 @@
 #include "zend_execute.h"
 #include "zend_vm.h"
 
+#define TO_STRING_NOWARN(val) do { \
+	if (Z_TYPE_P(val) < IS_ARRAY) { \
+		convert_to_string(val); \
+	} \
+} while (0)
+
 static void replace_by_const_or_qm_assign(zend_op_array *op_array, zend_op *opline, zval *result) {
 	if (opline->op1_type == IS_CONST) {
 		literal_dtor(&ZEND_OP1_LITERAL(opline));
@@ -64,10 +70,10 @@ void zend_optimizer_pass1(zend_op_array *op_array, zend_optimizer_ctx *ctx)
 		case ZEND_CONCAT:
 		case ZEND_FAST_CONCAT:
 			if (opline->op1_type == IS_CONST && Z_TYPE(ZEND_OP1_LITERAL(opline)) != IS_STRING) {
-				convert_to_string(&ZEND_OP1_LITERAL(opline));
+				TO_STRING_NOWARN(&ZEND_OP1_LITERAL(opline));
 			}
 			if (opline->op2_type == IS_CONST && Z_TYPE(ZEND_OP2_LITERAL(opline)) != IS_STRING) {
-				convert_to_string(&ZEND_OP2_LITERAL(opline));
+				TO_STRING_NOWARN(&ZEND_OP2_LITERAL(opline));
 			}
 			ZEND_FALLTHROUGH;
 		case ZEND_ADD:
@@ -100,7 +106,7 @@ void zend_optimizer_pass1(zend_op_array *op_array, zend_optimizer_ctx *ctx)
 		case ZEND_ASSIGN_OP:
 			if (opline->extended_value == ZEND_CONCAT && opline->op2_type == IS_CONST
 					&& Z_TYPE(ZEND_OP2_LITERAL(opline)) != IS_STRING) {
-				convert_to_string(&ZEND_OP2_LITERAL(opline));
+				TO_STRING_NOWARN(&ZEND_OP2_LITERAL(opline));
 			}
 			break;
 
diff --git a/Zend/tests/gh16408.phpt b/Zend/tests/gh16408.phpt
new file mode 100644
index 00000000000..f28c6435bfe
--- /dev/null
+++ b/Zend/tests/gh16408.phpt
@@ -0,0 +1,16 @@
+--TEST--
+GH-16408: Array to string conversion warning emitted in optimizer
+--FILE--
+<?php
+$counter = 0;
+ob_start(function ($buffer) use (&$c, &$counter) {
+        $c = 0;
+        ++$counter;
+}, 1);
+$c .= [];
+$c .= [];
+ob_end_clean();
+echo $counter . "\n";
+?>
+--EXPECT--
+3