PHP数组下标类型陷阱

项目使用PHP语言开发，其中用到了MONGO DB存储;MONGO DB里的数据是强类型，PHP里的数据是弱类型，上周五我在MONGODB里查询一个数据总是找不到，最后发现问题是PHP数组的数值型字符串下标自动转变成了整数型下标；因此虽然PHP是弱类型语言，我们也要关注变量当前什么类型，熟悉PHP的类型自动转换规则，在一些类型敏感的地方要进行类型判断或者强制类型转换。
 
    以下示例程序简单解释了这个现象：
 
 
 
Php代码
$id = "22";  
$arr1[$id] = "xxx";  
var_dump($arr1);  
$id = 22;  
$arr2[$id] = "xxx";  
var_dump($arr2);  
$id = "022";  
$arr3[$id] = "xxx";  
var_dump($arr3);  
$id = "2222222222222";  
$arr4[$id] = "xxx";  
var_dump($arr4); 
$id = "22";$arr1[$id] = "xxx";var_dump($arr1);$id = 22;$arr2[$id] = "xxx";var_dump($arr2);$id = "022";$arr3[$id] = "xxx";var_dump($arr3);$id = "2222222222222";$arr4[$id] = "xxx";var_dump($arr4);
    这段程序的输出是：
 
 
 
Php代码
array(1) {  
  [22]=>  
  string(3) "xxx" 
}  
array(1) {  
  [22]=>  
  string(3) "xxx" 
}  
array(1) {  
  ["022"]=>  
  string(3) "xxx" 
}  
array(1) {  
  ["2222222222222"]=>  
  string(3) "xxx" 
} 
array(1) {  [22]=>  string(3) "xxx"}array(1) {  [22]=>  string(3) "xxx"}array(1) {  ["022"]=>  string(3) "xxx"}array(1) {  ["2222222222222"]=>  string(3) "xxx"}
 
 
    那么，PHP的数组字符串下标类型是怎么确定的呢？我们一起到PHP的源代码里看一看。
 
    首先，我们在Zend/zend_language_parser.y里搜索[,找到数组的语义解析规则：
 
 
 
Php代码
object_dim_list:  
        object_dim_list '[' dim_offset ']'  { fetch_array_dim(&$$, &$1, &$3 TSRMLS_CC); }                           
    |   object_dim_list '{' expr '}'        { fetch_string_offset(&$$, &$1, &$3 TSRMLS_CC); }  
    |   variable_name { znode tmp_znode;  zend_do_pop_object(&tmp_znode TSRMLS_CC);  zend_do_fetch_property(&$$,  &tmp_znode, &$1 TSRMLS_CC);}     
; 
object_dim_list:        object_dim_list '[' dim_offset ']'  { fetch_array_dim(&$$, &$1, &$3 TSRMLS_CC); }                             |   object_dim_list '{' expr '}'        { fetch_string_offset(&$$, &$1, &$3 TSRMLS_CC); }    |   variable_name { znode tmp_znode;  zend_do_pop_object(&tmp_znode TSRMLS_CC);  zend_do_fetch_property(&$$,  &tmp_znode, &$1 TSRMLS_CC);}   ;
 
 
   我们使用的是数组，因此使用第一个规则fetch_array_dim,在fetch_array_dim函数里，我们发现生成的opcode是ZEND_FETCH_DIM_W(84)。在Zend/zend_vm_def.h里,ZEND_FETCH_DIM_W的处理函数里zend_fetch_dimension_address处理取下标逻辑。
 
 
 
    继续跟踪下去，从zend_fetch_dimension_address函数到zend_fetch_dimension_address_inner，再到zend_symtable_update：
 
 
 
Php代码
static inline int zend_symtable_update(HashTable *ht, char *arKey, uint nKeyLength, void *pData, uint nDataSize,  void **pDest)                 \  
{   
    HANDLE_NUMERIC(arKey, nKeyLength, zend_hash_index_update(ht, idx, pData, nDataSize, pDest));  
    return zend_hash_update(ht, arKey, nKeyLength, pData, nDataSize, pDest);                                        
}  
static inline int zend_symtable_update(HashTable *ht, char *arKey, uint nKeyLength, void *pData, uint nDataSize,  void **pDest)                 \{     HANDLE_NUMERIC(arKey, nKeyLength, zend_hash_index_update(ht, idx, pData, nDataSize, pDest));    return zend_hash_update(ht, arKey, nKeyLength, pData, nDataSize, pDest);                                      }
 
 
   HANDLE_NUMERIC这个宏很有意思，如果字符串下标arKey可转化为长整数idx,则调用zend_hash_index_update把数据插入到idx位置，否则调用zend_hash_update修改arKey位置的值 。我们看下宏的具体定义：
 
 
 
Php代码
#define HANDLE_NUMERIC(key, length, func) {                                             \  
    register char *tmp=key;                                                             \  
                                                                                        \  
    if (*tmp=='-') {                                                                    \  
        tmp++;                                                                          \  
    }                                                                                   \  
    if ((*tmp>='0' && *tmp         char *end=key+length-1;                                                         \  
        long idx;                                                                       \  
                                                                                        \  
        if (*tmp++=='0' && length>2) { /* don't accept numbers with leading zeros */    \  
            break;                                                                      \  
        }                                                                               \  
        while (tmp             if (!(*tmp>='0' && *tmp                 break;                                                                  \  
            }                                                                           \  
            tmp++;                                                                      \  
        }                                                                               \  
        if (tmp==end && *tmp=='\0') { /* a numeric index */                             \  
            if (*key=='-') {                                                            \  
                idx = strtol(key, NULL, 10);                                            \  
                if (idx!=LONG_MIN) {                                                    \  
                    return func;                                                        \  
                }                                                                       \  
            } else {                                                                    \  
                idx = strtol(key, NULL, 10);                                            \  
                if (idx!=LONG_MAX) {                                                    \  
                    return func;                                                        \  
                }                                                                       \  
            }                                                                           \  
        }                                                                               \  
    } while (0);                                                                        \  
} 
#define HANDLE_NUMERIC(key, length, func) {                                             \    register char *tmp=key;                                                             \                                                                                        \    if (*tmp=='-') {                                                                    \        tmp++;                                                                          \    }                                                                                   \    if ((*tmp>='0' && *tmp2) { /* don't accept numbers with leading zeros */    \            break;                                                                      \        }                                                                               \        while (tmp='0' && *tmp     从宏里我们知道了字符串下标自动转化为长整数下标的规则：
 
    1. 全部为数字，但是不能有前导0，比如arKey="0123"不会转化成123
 
    2. 不能超过long的表示范围(LONG_MIN, LONG_MAX)，即(-2147483648, 2147483647)