unique number from a string - php - php

Question

I have some strings containing alpha numeric values, say
asdf1234,
qwerty//2345
etc..
I want to generate a specific constant number related with the string. The number should not match any number generated corresponding with other string..

Answer 1

Does it have to be a number?
You could simply hash the string, which would give you a unique value.
echo md5('any string in here');
Note: This is a one-way hash, it cannot be converted from the hash back to the string.
This is how passwords are typically stored (using this or another hash function, typically with a 'salt' method added.) Checking a password is then done by hashing the input and comparing to the stored hash.
edit: md5 hashes are 32 characters in length.
Take a look at other hash functions:
http://us3.php.net/manual/en/function.crc32.php (returns a number, possibly negative)
http://us3.php.net/manual/en/function.sha1.php (40 characters)

Answer 2

You can use a hashing function like md5, but that's not very interesting.
Instead, you can turn the string into its sequence of ASCII characters (since you said that it's alpha-numeric) - that way, it can easily be converted back, corresponds to the string's length (length*3 to be exact), it has 0 collision chance, since it's just turning it to another representation, always a number and it's a little more interesting... Example code:
function encode($string) {
$ans = array();
$string = str_split($string);
#go through every character, changing it to its ASCII value
for ($i = 0; $i < count($string); $i++) {
#ord turns a character into its ASCII values
$ascii = (string) ord($string[$i]);
#make sure it's 3 characters long
if (strlen($ascii) < 3)
$ascii = '0'.$ascii;
$ans[] = $ascii;
}
#turn it into a string
return implode('', $ans);
}
function decode($string) {
$ans = '';
$string = str_split($string);
$chars = array();
#construct the characters by going over the three numbers
for ($i = 0; $i < count($string); $i+=3)
$chars[] = $string[$i] . $string[$i+1] . $string[$i+2];
#chr turns a single integer into its ASCII value
for ($i = 0; $i < count($chars); $i++)
$ans .= chr($chars[$i]);
return $ans;
}
Example:
$original = 'asdf1234';
#will echo
#097115100102049050051052
$encoded = encode($original);
echo $encoded . "\n";
#will echo asdf1234
$decoded = decode($encoded);
echo $decoded . "\n";
echo $original === $decoded; #echoes 1, meaning true

Answer 3

You're looking for a hash function, such as md5. You probably want to pass it the $raw_output=true parameter to get access to the raw bytes, then cast them to whatever representation you want the number in.

Answer 4

A cryptographic hash function will give you a different number for each input string, but it's a rather large number — 20 bytes in the case of SHA-1, for example. In principle it's possible for two strings to produce the same hash value, but the chance of it happening is so extremely small that it's considered negligible.
If you want a smaller number — say, a 32-bit integer — then you can't use a hash function because the probability of collision is too high. Instead, you'll need to keep a record of all the mappings you've established. Make a database table that associates strings with numbers, and each time you're given a string, look it up in the table. If you find it there, return the associated number. If not, choose a new number that isn't used by any of the existing records, and add the new string and number to the table.