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
} \
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
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)

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