diff --git a/Zend/zend_execute.c b/Zend/zend_execute.c
index 733ce54dc24ad..c0097c55cbc48 100644
--- a/Zend/zend_execute.c
+++ b/Zend/zend_execute.c
@@ -4688,6 +4688,44 @@ static const zend_live_range *find_live_range(const zend_op_array *op_array, uin
 }
 /* }}} */
 
+/* Replace optional parameters that weren't passed with their declared default values,
+ * which allows us to check that this does not change the behavior of the function. */
+#define ZEND_VERIFY_INTERNAL_PARAM_DEFAULTS 1
+#if ZEND_VERIFY_INTERNAL_PARAM_DEFAULTS
+static void zend_verify_internal_param_defaults(zend_execute_data **call_ptr) {
+	zend_function *fbc = (*call_ptr)->func;
+	uint32_t num_passed_args = ZEND_CALL_NUM_ARGS(*call_ptr);
+	if (num_passed_args < fbc->common.required_num_args) {
+		/* This is an error anyway. */
+		return;
+	}
+
+	uint32_t num_declared_args = fbc->common.num_args;
+	while (num_passed_args < num_declared_args) {
+		zend_internal_arg_info *arg_info = &fbc->internal_function.arg_info[num_passed_args];
+		zval default_value;
+		if (zend_get_default_from_internal_arg_info(&default_value, arg_info) == FAILURE) {
+			/* Default value not available, so we can't pass any further defaults either. */
+			return;
+		}
+
+		if (Z_TYPE(default_value) == IS_CONSTANT_AST) {
+			zval_update_constant_ex(&default_value, fbc->common.scope);
+		}
+
+		zend_vm_stack_extend_call_frame(call_ptr, num_passed_args, 1);
+		zval *arg = ZEND_CALL_VAR_NUM(*call_ptr, num_passed_args);
+		ZVAL_COPY_VALUE(arg, &default_value);
+		if (ARG_SHOULD_BE_SENT_BY_REF(fbc, num_passed_args + 1)) {
+			ZVAL_MAKE_REF(arg);
+		}
+
+		num_passed_args++;
+		ZEND_CALL_NUM_ARGS(*call_ptr)++;
+	}
+}
+#endif
+
 static void cleanup_live_vars(zend_execute_data *execute_data, uint32_t op_num, uint32_t catch_op_num) /* {{{ */
 {
 	int i;
diff --git a/Zend/zend_vm_def.h b/Zend/zend_vm_def.h
index 55fdb7d46582b..be3a17da5c96c 100644
--- a/Zend/zend_vm_def.h
+++ b/Zend/zend_vm_def.h
@@ -4046,6 +4046,9 @@ ZEND_VM_HOT_HANDLER(129, ZEND_DO_ICALL, ANY, ANY, SPEC(RETVAL,OBSERVER))
 	zval retval;
 
 	SAVE_OPLINE();
+#if ZEND_VERIFY_INTERNAL_PARAM_DEFAULTS
+zend_verify_internal_param_defaults(&call);
+#endif
 	EX(call) = call->prev_execute_data;
 
 	call->prev_execute_data = execute_data;
diff --git a/Zend/zend_vm_execute.h b/Zend/zend_vm_execute.h
index 16455b6e0cd58..55037e618ffe3 100644
--- a/Zend/zend_vm_execute.h
+++ b/Zend/zend_vm_execute.h
@@ -1272,6 +1272,9 @@ static ZEND_VM_HOT ZEND_OPCODE_HANDLER_RET ZEND_FASTCALL ZEND_DO_ICALL_SPEC_RETV
 	zval retval;
 
 	SAVE_OPLINE();
+#if ZEND_VERIFY_INTERNAL_PARAM_DEFAULTS
+zend_verify_internal_param_defaults(&call);
+#endif
 	EX(call) = call->prev_execute_data;
 
 	call->prev_execute_data = execute_data;
@@ -1334,6 +1337,9 @@ static ZEND_VM_HOT ZEND_OPCODE_HANDLER_RET ZEND_FASTCALL ZEND_DO_ICALL_SPEC_RETV
 	zval retval;
 
 	SAVE_OPLINE();
+#if ZEND_VERIFY_INTERNAL_PARAM_DEFAULTS
+zend_verify_internal_param_defaults(&call);
+#endif
 	EX(call) = call->prev_execute_data;
 
 	call->prev_execute_data = execute_data;
@@ -1396,6 +1402,9 @@ static ZEND_VM_COLD ZEND_OPCODE_HANDLER_RET ZEND_FASTCALL ZEND_DO_ICALL_SPEC_OBS
 	zval retval;
 
 	SAVE_OPLINE();
+#if ZEND_VERIFY_INTERNAL_PARAM_DEFAULTS
+zend_verify_internal_param_defaults(&call);
+#endif
 	EX(call) = call->prev_execute_data;
 
 	call->prev_execute_data = execute_data;