This RFC allows writing to object properties (and isset() / unset() on properties) when the object instance is referenced by a constant.
PHP constants are immutable in the sense that the constant binding cannot be changed. However, objects in PHP are mutable by design: mutating an object property does not rebind the constant, it only updates the state of the referenced object.
Today, PHP rejects CONST_OBJ->prop = ... with a fatal error (“Cannot use temporary expression in write context”).
This RFC proposes permitting property write operations in this specific scenario, aligning behavior with user expectations while keeping the constant binding immutable.
<?php const OBJ = new stdClass(); OBJ->value = 123; var_dump(OBJ->value); // int(123) unset(OBJ->value); var_dump(isset(OBJ->value)); // bool(false) ?>
This applies both to global constants (const / define()) and to class constants (Cls::CONST).
This RFC proposes to allow property write operations where the base expression is a constant that evaluates to an object.
The change is intentionally narrow:
[]), nor does it change “write-to-temporary” behavior for dims.CONST->prop = ...) and class constants (Cls::CONST->prop = ... and $obj::CONST->prop = ...).Specifically, the following operations become valid:
<?php const OBJ = new stdClass(); OBJ->prop = "value"; ?>
<?php const OBJ = new stdClass(); OBJ->counter = 0; OBJ->counter++; OBJ->counter--; OBJ->counter += 10; OBJ->str = 'hello'; OBJ->str .= ' world'; ?>
<?php const OBJ = new stdClass(); OBJ->a = new stdClass(); OBJ->a->b = 7; var_dump(OBJ->a->b); // int(7) ?>
<?php const OBJ = new stdClass(); OBJ->val = 10; function modify(&$v) { $v = 42; } modify(OBJ->val); var_dump(OBJ->val); // int(42) ?>
A related but distinct scenario exists with array constants:
<?php const ARR = [1, 2, 3]; $ref = ARR; $ref[0] = 9; var_dump(ARR); // [1, 2, 3] — original unchanged ?>
Because arrays use copy-on-write semantics, modifying the copy has no effect on the original constant. Similarly:
<?php const C = [1, 2, 3]; function foo(): array { return C; } foo()[] = 4; var_dump(C); // [1, 2, 3] — unmodified, assignment is on a temporary ?>
These dim-write cases involve assignment to temporaries that silently have no effect. Whether such no-effect writes should produce a warning or error is a valid concern but is out of scope for this RFC and may be addressed separately (e.g. warning/throwing on “ASSIGN_DIM” when OP1 is not a pointer — i.e. not an indirect, reference, or object — as the operation can never have an effect). See the Future Scope section.
This RFC does NOT change:
<?php const OBJ = new stdClass(); OBJ = new stdClass(); // still illegal (parse error) ?>
<?php const ARR = [1,2,3]; ARR[0] = 9; // unchanged (still rejected as before) const OBJ = new stdClass(); OBJ['x'] = 1; // unchanged (still rejected as before) ?>
The compiler marks the constant AST node rather than emitting the fetch directly. zend_delayed_compile_prop() in Zend/zend_compile.c calls zend_mark_const_object_fetch(), which walks down through any ZEND_AST_DIM nodes and sets a ZEND_CONST_OBJECT_FETCH attribute on a ZEND_AST_CONST or ZEND_AST_CLASS_CONST base. zend_delayed_compile_var() then handles those two AST kinds when the attribute is set, so the constant is fetched at runtime and the property write targets the real object.
zend_emit_fetch_constant() and zend_compile_class_const() take the fetch type: in read context (BP_VAR_R, BP_VAR_IS) the result stays IS_TMP_VAR as before, and in write context it is IS_VAR. Because a constant may still be substituted at compile time (true, null, persistent constants, same-file class constants), a compile-time-evaluated constant in write context is wrapped in a ZEND_QM_ASSIGN to materialise it into an IS_VAR; the property opcodes have no handler for an IS_CONST container operand.
Two supporting changes were needed. zend_optimizer_update_op1_const() now refuses to substitute a constant into op1 of the object-write opcodes (ZEND_ASSIGN_OBJ, ZEND_FETCH_OBJ_W, ZEND_UNSET_OBJ, ZEND_PRE_INC_OBJ and friends), which would otherwise reintroduce the invalid operand during optimisation. And zend_can_write_to_variable() accepts a constant at the base of a PROP/DIM chain when the chain contains at least one property fetch, so that destructuring assignment and by-reference foreach can target a constant object property; a write to the constant itself, or to a dimension of it, still targets a temporary and remains illegal.
This feature addresses a common surprise: PHP already permits objects in constants (since PHP 8.1), and developers naturally expect to mutate the object's state through the constant reference. The current fatal error is not about constant immutability per se, but rather about an overly broad “temporary write context” restriction in the compiler. Narrowly enabling property writes reduces friction, improves ergonomics, and matches the runtime model of objects without increasing language complexity for other write contexts.
Enum cases are class constants, so this change affects them. Writing to or unsetting a readonly case property — for example unset(Enum::Case->value) or Enum::Case->value = $x — now raises the standard runtime readonly-property “Error” (“Cannot unset/modify readonly property”) instead of the previous compile-time fatal (“Cannot use temporary expression in write context”).
This makes enum-case property access consistent with plain variable access ($x = Enum::Case;
unset($x->value);). Enum case properties remain immutable — no value that was previously read-only becomes writable, and dynamic properties on enum cases continue to be rejected.
Apart from this, code that previously terminated with a fatal error for CONST_OBJ->prop = ... will now execute successfully. This is a compatibility improvement — no previously-working code changes behavior.
PHP 8.6
No expected impact. This is a core compilation change around property write access, not an extension API change.
No expected impact.
None currently.
foo()[0] = 1 where the return value is a temporary are similarly out of scope but could be revisited.Primary Vote requiring a 2/3 majority to accept the RFC:
Proof of concept implementation:
Cls::CONST->prop and $obj::CONST->prop) per Claude Pache's feedback. Documented the enum readonly-property backward-incompatible change. Restored the Abstain vote option (added by Tim Düsterhus). Restarted the discussion period for the widened scope.Key changes from v0.1: - Added by-ref passing examples (confirmed working in tests) - Added implementation details section explaining the compiler change - Added “Note on Array Constants” section addressing Ilija's and Tim's feedback about silent no-op dim writes - Expanded Future Scope with Tim's suggestion about warning on ASSIGN_DIM with non-pointer OP1 - Voting options were revised during later RFC updates; the current vote includes Yes, No, and Abstain - Removed the placeholder voting close date until voting opens - Targeted PHP 8.6 explicitly - Listed all test files in the Patches and Tests section - Removed Open Issues (the by-ref question is answered, isset/unset is confirmed) - Added full URLs to References - Added changelog entry for v0.2