Language: String offset access and assignment ($str[$i])

[PurHur]

Problem

Byte-level string manipulation ($slug[0], mutating chars) is common in routers and sanitizers. Compiler may not lower string dim-fetch/write uniformly across VM/JIT.

Goal

Read/write single-byte offsets on PHP strings (Zend-compatible for ASCII subset).

Tasks

• Audit TYPE_ARRAY_DIM_FETCH for string operands in lib/Compiler.php
• VM: bounds checks, copy-on-write for string updates
• JIT: native byte load/store on JitNativeString / string type
• PHPT: read, assign, out-of-range behavior
• Document UTF-8 limitation (multibyte = Stdlib: mb_strlen and UTF-8 byte-safe string metrics #158)

Acceptance criteria

$s = 'abc';
$s[1] = 'z';
echo $s; // azc

Files

• lib/Compiler.php, lib/VM.php, lib/JIT.php, lib/JIT/JitNativeString.php