fix(php): 修复数组维度赋值解析逻辑

- 简化了 isArrayDimFetch 条件判断，添加 isVarExpr 验证
- 重构了字符串类型检查逻辑，移除冗余条件判断
- 直接返回 parseAssignArrayDim 调用结果，简化代码流程
- 移除了不必要的变量赋值和条件分支

src/Php/CompilerBase.php

@@ -1443,16 +1443,12 @@ class CompilerBase extends \PhpAot\Core\Translator
             }
         } elseif ($this->isPropertyFetch($left) and !$left->getAttribute('nativeProperty')) {
             return $this->parseAssignPropertyFetch($left, $right);
-        } elseif ($this->isArrayDimFetch($left)) {
+        } elseif ($this->isArrayDimFetch($left) and $this->isVarExpr($left->var)) {
             $tmp = $this->parseIdentifier($left->var);
-            if ($this->isVarExpr($left->var) and $this->getVarType($tmp) === self::TYPE_STR) {
-                if ($left->dim === null) {
-                    $this->fatalError($left, 'Cannot use [] for strings');
-                }
-            }
-            if ($this->isVarExpr($left->var) and ($this->isVarExpr($right) or $this->isScalar($right))) {
-                return $this->parseAssignArrayDim($left, $right);
+            if ($this->getVarType($tmp) === self::TYPE_STR and $left->dim === null) {
+                $this->fatalError($left, 'Cannot use [] for strings');
             }
+            return $this->parseAssignArrayDim($left, $right);
         }
 
         return $var . ' = ' . $this->convertExprType($expr, $this->detectExprType($left), $this->detectExprType($right));