How to create a random string using PHP?

If you want to allow repetitive occurences of characters, you can use this function:

function randString($length, $charset='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'){    $str = '';    $count = strlen($charset);    while ($length--) {        $str .= $charset[mt_rand(0, $count-1)];    }    return $str;}

The basic algorithm is to generate <length> times a random number between 0 and <number of characters> − 1 we use as index to pick a character from our set and concatenate those characters. The 0 and <number of characters> − 1 bounds represent the bounds of the $charset string as the first character is addressed with $charset[0] and the last with $charset[count($charset) - 1].

Well, you didn't clarify all the questions I asked in my comment, but I'll assume that you want a function that can take a string of "possible" characters and a length of string to return. Commented thoroughly as requested, using more variables than I would normally, for clarity:

function get_random_string($valid_chars, $length){    // start with an empty random string    $random_string = "";    // count the number of chars in the valid chars string so we know how many choices we have    $num_valid_chars = strlen($valid_chars);    // repeat the steps until we've created a string of the right length    for ($i = 0; $i < $length; $i++)    {        // pick a random number from 1 up to the number of valid chars        $random_pick = mt_rand(1, $num_valid_chars);        // take the random character out of the string of valid chars        // subtract 1 from $random_pick because strings are indexed starting at 0, and we started picking at 1        $random_char = $valid_chars[$random_pick-1];        // add the randomly-chosen char onto the end of our string so far        $random_string .= $random_char;    }    // return our finished random string    return $random_string;}

To call this function with your example data, you'd call it something like:

$original_string = 'abcdefghi';$random_string = get_random_string($original_string, 6);

Note that this function doesn't check for uniqueness in the valid chars passed to it. For example, if you called it with a valid chars string of 'AAAB', it would be three times more likely to choose an A for each letter as a B. That could be considered a bug or a feature, depending on your needs.

My favorite:

 echo substr(md5(rand()), 0, 7);