md5()

(PHP 4, PHP 5, PHP 7)

计算字符串的 MD5 散列值

It is not recommended to use this function to secure passwords, due to the fast nature of this hashing algorithm. See thePassword Hashing FAQfor details and best practices.

说明

使用» RSA 数据安全公司的 MD5 报文算法计算$str的 MD5 散列值。

参数

原始字符串。

如果可选的$raw_output被设置为TRUE，那么 MD5 报文摘要将以16字节长度的原始二进制格式返回。

返回值

以 32 字符十六进制数字形式返回散列值。

范例

Example #1md5()范例

参见

• md5_file()计算指定文件的 MD5 散列值
• sha1_file()计算文件的 sha1 散列值
• crc32()计算一个字符串的 crc32 多项式
• sha1()计算字符串的 sha1 散列值
• hash()生成哈希值（消息摘要）
• crypt()单向字符串散列
• password_hash()创建密码的散列（hash）

md5('240610708') == md5('QNKCDZO')
This comparison is true because both md5() hashes start '0e' so PHP type juggling understands these strings to be scientific notation. By definition, zero raised to any power is zero.

Use the strict comparision/identity operator:
php > var_dump(md5('240610708') == md5('QNKCDZO'));
php shell code:1:
bool(true)
php > var_dump(md5('240610708') === md5('QNKCDZO'));
php shell code:1:
bool(false)
php >
because, as mentioned previously, using the equality operator may result in false positives.

The complement of hey2raw.
You can use to convert from raw md5-format to human-readable format.
This can be usefull to check "Content-Md5" HTTP-Header.

From the documentation on Digest::MD5:
md5($data,...)
This function will concatenate all arguments, calculate the MD5 digest of this "message", and return it in binary form.
md5_hex($data,...)
Same as md5(), but will return the digest in hexadecimal form. 
PHP's function returns the digest in hexadecimal form, so my guess is that you're using md5() instead of md5_hex(). I have verified that md5_hex() generates the same string as PHP's md5() function.
(original comment snipped in various places)
>Hexidecimal hashes generated with Perl's Digest::MD5 module WILL
>NOT equal hashes generated with php's md5() function if the input
>text contains any non-alphanumeric characters.
>
>$phphash = md5('pa$$');
>echo "php original hash from text: $phphash";
>echo "md5 hash from perl: " . $myrow['password'];
>
>outputs:
>
>php original hash from text: 0aed5d740d7fab4201e885019a36eace
>hash from perl: c18c9c57cb3658a50de06491a70b75cd

Note: Before you get some idea like using md5 with password as way to prevent others tampering with message, read pages "Length extension attack" and "Hash-based message authentication code" on wikipedia. In short, naive constructions can be dangerously insecure. Use hash_hmac if available or reimplement HMAC properly without shortcuts.

Do not use the hex strings returned by md5() as a key for MCrypt 256-bit encryption. Hex characters only represent four bits each, so when you take 32 hex characters, you are only really using a 128-bit key, not a 256-bit one. 
Using an alphanumeric key generator [A-Za-z0-9] will also only provide a 192-bit key in 32 characters.
Two different MD5s concatenated in raw binary form, or mcrypt_create_iv(32,MCRYPT_DEV_RANDOM) will give you a true 256-bit key string.

Sometimes it's useful to get the actual, binary, md5 digest. 
You can use this function for it:

If you want to hash a large amount of data you can use the hash_init/hash_update/hash_final functions.
This allows you to hash chunks/parts/incremental or whatever you like to call it.

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