feat(php): 添加字符串数组索引赋值支持

- 实现字符串类型的数组维度获取解析功能
- 添加字符串索引赋值的类型检测和错误处理
- 支持字符串的 offsetSet 操作转换
- 新增字符串数组索引赋值的功能测试用例
- 处理字符串索引为 null 时的错误情况
- 实现字符串变量类型检查机制

src/Php/CompilerBase.php

@@ -1079,6 +1079,15 @@ class CompilerBase extends \PhpAot\Core\Translator
             }
         } elseif ($this->isPropertyFetch($left)) {
             $var = $this->parsePropertyFetch($left, true);
+        } elseif ($this->isArrayDimFetch($left)) {
+            $tmp = $this->parseIdentifier($left->var);
+            var_dump($tmp);
+            if ($this->isVarExpr($left->var) and $this->getVarType($tmp) === self::TYPE_STR) {
+                if ($left->dim === null) {
+                    $this->fatalError($left, 'Cannot use [] for strings');
+                }
+                return $tmp . '.offsetSet(' . $this->parseExpr($left->dim) . ', ' . $this->convertExprType($expr, $this->detectExprType($left), $this->detectExprType($right)) . ')';
+            }
         }
 
         return $var . ' = ' . $this->convertExprType($expr, $this->detectExprType($left), $this->detectExprType($right));
@@ -1689,6 +1698,11 @@ class CompilerBase extends \PhpAot\Core\Translator
                     $this->fatalError($node->var, "The variable `{$node->var->name}` is undefined");
                 }
             }
+            if ($this->getVarType($var) === self::TYPE_STR) {
+                if ($node->dim === null) {
+                    $this->fatalError($node, 'Cannot use [] for strings');
+                }
+            }
         }
 
         if ($node->dim === null) {

tests/aot/str-offset-set.phpt

@@ -0,0 +1,12 @@
+--TEST--
+strlen
+--FILE--
+<?php
+$s1 = "hello world";
+$s2 = "php";
+$s1[1] = $s2[1] = '_';
+var_dump($s1, $s2);
+?>
+--EXPECT--
+string(11) "h_llo world"
+string(3) "p_p"