diff --git a/Zend/Optimizer/optimize_func_calls.c b/Zend/Optimizer/optimize_func_calls.c
index 154baddc65426..c54c531bcfc1a 100644
--- a/Zend/Optimizer/optimize_func_calls.c
+++ b/Zend/Optimizer/optimize_func_calls.c
@@ -38,6 +38,7 @@
 typedef struct _optimizer_call_info {
 	zend_function *func;
 	zend_op       *opline;
+	zend_op       *last_check_func_arg_opline;
 	bool      is_prototype;
 	bool      try_inline;
 	uint32_t       func_arg_num;
@@ -252,6 +253,14 @@ void zend_optimize_func_calls(zend_op_array *op_array, zend_optimizer_ctx *ctx)
 				if (call_stack[call - 1].func_arg_num != (uint32_t)-1
 						&& has_known_send_mode(&call_stack[call - 1], call_stack[call - 1].func_arg_num)) {
 					if (ARG_SHOULD_BE_SENT_BY_REF(call_stack[call - 1].func, call_stack[call - 1].func_arg_num)) {
+						/* There's no TMP specialization for FETCH_OBJ_W/FETCH_DIM_W. Avoid
+						 * converting it and error at runtime in the FUNC_ARG variant. */
+						if ((opline->opcode == ZEND_FETCH_OBJ_FUNC_ARG || opline->opcode == ZEND_FETCH_DIM_FUNC_ARG)
+						 && (opline->op1_type == IS_TMP_VAR || call_stack[call - 1].last_check_func_arg_opline == NULL)) {
+							/* Don't remove the associated CHECK_FUNC_ARG opcode. */
+							call_stack[call - 1].last_check_func_arg_opline = NULL;
+							break;
+						}
 						if (opline->opcode != ZEND_FETCH_STATIC_PROP_FUNC_ARG) {
 							opline->opcode -= 9;
 						} else {
@@ -298,11 +307,21 @@ void zend_optimize_func_calls(zend_op_array *op_array, zend_optimizer_ctx *ctx)
 
 				if (has_known_send_mode(&call_stack[call - 1], opline->op2.num)) {
 					call_stack[call - 1].func_arg_num = opline->op2.num;
-					MAKE_NOP(opline);
+					call_stack[call - 1].last_check_func_arg_opline = opline;
 				}
 				break;
-			case ZEND_SEND_VAR_EX:
 			case ZEND_SEND_FUNC_ARG:
+				/* Don't transform SEND_FUNC_ARG if any FETCH opcodes weren't transformed. */
+				if (call_stack[call - 1].last_check_func_arg_opline == NULL) {
+					if (opline->op2_type == IS_CONST) {
+						call_stack[call - 1].try_inline = 0;
+					}
+					break;
+				}
+				MAKE_NOP(call_stack[call - 1].last_check_func_arg_opline);
+				call_stack[call - 1].last_check_func_arg_opline = NULL;
+				ZEND_FALLTHROUGH;
+			case ZEND_SEND_VAR_EX:
 				if (opline->op2_type == IS_CONST) {
 					call_stack[call - 1].try_inline = 0;
 					break;
diff --git a/Zend/tests/gh12102_1.phpt b/Zend/tests/gh12102_1.phpt
new file mode 100644
index 0000000000000..1d548d6b28f3c
--- /dev/null
+++ b/Zend/tests/gh12102_1.phpt
@@ -0,0 +1,30 @@
+--TEST--
+GH-12102: Incorrect "Cannot use temporary expression in write context" error for BP_VAR_FUNC_ARG
+--FILE--
+<?php
+
+function test() {
+    byVal(func_get_args()[0]);
+    try {
+        byRef(func_get_args()[0]);
+    } catch (Error $e) {
+        echo $e->getMessage(), "\n";
+    }
+}
+
+/* Intentionally declared after test() to avoid compile-time checking of ref args. */
+
+function byVal($arg) {
+    var_dump($arg);
+}
+
+function byRef(&$arg) {
+    var_dump($arg);
+}
+
+test('y');
+
+?>
+--EXPECT--
+string(1) "y"
+Cannot use temporary expression in write context
diff --git a/Zend/tests/gh12102_2.phpt b/Zend/tests/gh12102_2.phpt
new file mode 100644
index 0000000000000..dacc11c03081a
--- /dev/null
+++ b/Zend/tests/gh12102_2.phpt
@@ -0,0 +1,43 @@
+--TEST--
+GH-12102: Incorrect "Cannot use temporary expression in write context" error for BP_VAR_FUNC_ARG
+--FILE--
+<?php
+
+function test() {
+    global $ref;
+    byVal(getRef()[0]);
+    var_dump($ref);
+    byRef(getRef()[0]);
+    var_dump($ref);
+}
+
+/* Intentionally declared after test() to avoid compile-time checking of ref args. */
+
+function &getRef() {
+    global $ref;
+    $ref = [];
+    return $ref;
+}
+
+function byVal($arg) {
+    $arg[] = 42;
+}
+
+function byRef(&$arg) {
+    $arg[] = 42;
+}
+
+test();
+
+?>
+--EXPECTF--
+Warning: Undefined array key 0 in %s on line %d
+array(0) {
+}
+array(1) {
+  [0]=>
+  array(1) {
+    [0]=>
+    int(42)
+  }
+}
diff --git a/Zend/tests/gh12102_3.phpt b/Zend/tests/gh12102_3.phpt
new file mode 100644
index 0000000000000..741bce5ab1ba1
--- /dev/null
+++ b/Zend/tests/gh12102_3.phpt
@@ -0,0 +1,32 @@
+--TEST--
+GH-12102: Incorrect "Cannot use temporary expression in write context" error for BP_VAR_FUNC_ARG
+--FILE--
+<?php
+
+function test() {
+    byVal(C[0]);
+    try {
+        byRef(C[0]);
+    } catch (Error $e) {
+        echo $e->getMessage(), "\n";
+    }
+}
+
+/* Intentionally declared after test() to avoid compile-time checking of ref args. */
+
+const C = ['foo'];
+
+function byVal($arg) {
+    var_dump($arg);
+}
+
+function byRef(&$arg) {
+    var_dump($arg);
+}
+
+test('y');
+
+?>
+--EXPECT--
+string(3) "foo"
+Cannot use temporary expression in write context
diff --git a/Zend/zend_compile.c b/Zend/zend_compile.c
index 75127b8b8d18a..c0cd3c97341f9 100644
--- a/Zend/zend_compile.c
+++ b/Zend/zend_compile.c
@@ -2816,7 +2816,11 @@ static zend_op *zend_compile_simple_var(znode *result, zend_ast *ast, uint32_t t
 
 static void zend_separate_if_call_and_write(znode *node, zend_ast *ast, uint32_t type) /* {{{ */
 {
-	if (type != BP_VAR_R && type != BP_VAR_IS && zend_is_call(ast)) {
+	if (type != BP_VAR_R
+	 && type != BP_VAR_IS
+	 /* Whether a FUNC_ARG is R may only be determined at runtime. */
+	 && type != BP_VAR_FUNC_ARG
+	 && zend_is_call(ast)) {
 		if (node->op_type == IS_VAR) {
 			zend_op *opline = zend_emit_op(NULL, ZEND_SEPARATE, node, NULL);
 			opline->result_type = IS_VAR;