What is the best way to create a random hash/string?

random_bytes() is available as of PHP 7.0 (or use this polyfill for 5.2 through 5.6). It is cryptographically secure (compared to rand() which is not) and can be used in conjunction with bin2hex(), base64_encode(), or any other function that converts binary to a string that's safe for your use case.

As a hexadecimal string

bin2hex() will result in a hexadecimal string that's twice as many characters as the number of random bytes (each hex character represents 4 bits while there are 8 bits in a byte). It will only include characters from abcdef0123456789 and the length will always be an increment of 2 (regex: /^([a-f0-9]{2})*$/).

$random_hex = bin2hex(random_bytes(18));
echo serialize($random_hex);

s:36:"ee438d1d108bd818aa0d525602340e5d7036";

As a base64 string

base64_encode() will result in a string that's about 33% longer than the number of random bytes (each base64 character represents 6 bits while there are 8 bits in a byte). It's length will always be an increment of 4, with = used to pad the end of the string and characters from the following list used to encode the data (excluding whitespace that I added for readability):

abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ
0123456789
/+

To take full advantage of the space available, it's best to provide an increment of 3 to random_bytes(). The resulting string will match /^([a-zA-Z\/+=]{4})*$/, although = can only appear at the end as = or == and only when a number that is not an increment of 3 is provided to random_bytes().

$random_base64 = base64_encode(random_bytes(18));
echo serialize($random_base64);

s:24:"ttYDDiGPV5K0MXbcfeqAGniH";

Maybe uniqid() is what you need?

uniqid — Generate a unique ID

You can use PHP's built-in hashing functions, sha1 and md5. Choose one, not both.

One may think that using both, sha1(md5($pass)) would be a solution. Using both does not make your password more secure, its causes redundant data and does not make much sense.

Take a look at PHP Security Consortium: Password Hashing they give a good article with weaknesses and improving security with hashing.

Nonce stands for "numbers used once". They are used on requests to prevent unauthorized access, they send a secret key and check the key each time your code is used.

You can check out more at PHP NONCE Library from FullThrottle Development

bin2hex(mcrypt_create_iv(22, MCRYPT_DEV_URANDOM));

1. mcrypt_create_iv will give you a random sequence of bytes.
2. bin2hex will convert it to ASCII text

Example output:

d2c63a605ae27c13e43e26fe2c97a36c4556846dd3ef

Bare in mind that "best" is a relative term. You have a tradeoff to make between security, uniqueness and speed. The above example is good for 99% of the cases, though if you are dealing with a particularly sensitive data, you might want to read about the difference between MCRYPT_DEV_URANDOM and MCRYPT_DEV_RANDOM.

Finally, there is a RandomLib "for generating random numbers and strings of various strengths".

Notice that so far I have assumed that you are looking to generate a random string, which is not the same as deriving a hash from a value. For the latter, refer to password_hash.