spl_object_hash()

(PHP 5 >= 5.2.0, PHP 7)

返回指定对象的hash id

说明

本函数为指定对象返回一个唯一标识符。这个标识符可用于作为保存对象或区分不同对象的hash key。

参数

Any object.

返回值

字符串，对于每个对象它都是唯一的，并且对同一个对象它总是相同。

范例

A spl_object_hash() example

注释

When an object is destroyed, its hash may be reused for other objects.

Note that the contents (properties) of the object are NOT hashed by the function, merely its internal handle and handler table pointer. This is sufficient to guarantee that any two objects simultaneously co-residing in memory will have different hashes. Uniqueness is not guaranteed between objects that did not reside in memory simultaneously, for example:
var_dump(spl_object_hash(new stdClass()), spl_object_hash(new stdClass()));
Running this alone will usually generate the same hashes, since PHP reuses the internal handle for the first stdClass after it has been dereferenced and destroyed when it creates the second stdClass.

Note that given two different objects spl_object_hash() can return values that look very similar, and in fact both the most significant *and* least significant digits are likely to be identical! e.g. "000000003cc56d770000000007fa48c5" and "000000003cc56d0d0000000007fa48c5".
Therefore (especially if using this function for debugging), you may wish to pass the hash into a cryptographic hash function like md5() to get to facilitate visual comparisons, and make it more likely that the first few or last few digits are unique.
md5("000000003cc56d770000000007fa48c5") -> "619a799747d348fa1caf181a72b65d9f"
md5("000000003cc56d0d0000000007fa48c5") -> "ae964bc912281e7804fe5a88b4546cb2"

For those who believe this function is misnamed, I would like to direct you to https://en.wikipedia.org/wiki/Hash_function . Also, for those who think it's misnamed and supply a comparison to Python, I would like to direct you to https://docs.python.org/2/library/functions.html#hash which does the same thing as this function. (From Python's data-model docs: "User-defined classes have __cmp__() and __hash__() methods by default; ... x.__hash__() returns a result derived from id(x)." - id(x) returns the memory address of the object.)
The cryptographic hash functions you are familiar with, like MD5 or SHA1, are named hash functions because they have a similar design goal: low chance of collisions.

Please note that since PHP 7.2 there's new function available spl_object_id() which returns int instead of string. It's (supposed to be) more performant. Due to lack of documentation I recommend you reading the PR https://github.com/php/php-src/pull/2611

The "hash" mentioned in the name of this function refers to the storage structure known as a "hash table", not to any sort of "message digest". The string returned by this function is little more than the object's address in the (hash) table PHP maintains of all existing objects.

Calling this a hash is very misleading:
1. This function gives an object identifier (ID), which uniquely identifies the object for its whole lifetime. This is similar to the address of an object in C or the id() function in Python. I'm sure other languages have similar constructs.
2. This is not a hash and has nothing to do with it. A hash takes data and algorithmically reduces that data to some kind of scalar value. The only guarantee is that two equal inputs provide the same output, but not that two different inputs provide different outputs (hint: hash collisions). spl_object_hash() guarantees different outputs for non-identical objects though.
3. As someone mentioned already, this does not look at the content of the object. If you consider the difference between equality and identity, it only allows determining identity. If you serialize and unserialize an object, it will not be identical to its former self, but it will be equal, just to give an example. If you want a key to use in a response cache, using this function on the request is not only useless, because equal requests have different IDs, but possibly even harmful, because when a request object is garbage collected, its ID can be reused.

The "hash" mentioned in this function is used in the sense of the storage structure known as a "hash table", not in the sense of "message digest".

The "hash" mentioned in this function is used in the sense of the storage structure known as a "hash table", not in the sense of "message digest".

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